What’s Getting up Your Nose & Making You Sneeze

When to Plant Warm Season Veggies

Fernleaf Peony

The bright and bloomy month of May is here and, if you're like me, you've already got your hands way into the dirt. The fernleaf peony (shown above last week) is the first peony to bloom in my garden and has loads of character even before the flowers open. I’m eager to try some new veggie varieties this year, especially two that Larry Stebbins of Colorado Springs recommends so highly: dark green Raven zucchini and the prolific Corentine cucumber. His blog – the gardenfather.com/garden-blog – is a terrific source of vegetable gardening info.

I’ve been pulling weeds while recent moisture makes it easy, getting veggie seeds into the ground, and pruning roses and grapes. Those long gloves I won at the Garden Bloggers Fling a few years ago really come in handy.

In our print issues we always included the monthly moon phases so I’m reviving that tradition.

There’s a whole complex system of planting by the moon but the gist of it is to plant for below ground crops like beets and carrots while the moon is waning, and for above ground leafy growth and fruit while it’s waxing. I know of some local nurserymen who report much better germination rates, especially for difficult-to-propagate plants, when they take moon phases into account.

We have two new features for you this month. Indefatigably generous plantsman Panayoti Kelaidis writes about three categories of garden Phlox: the tall old cultivars that bloom in late summer, low bright groundcovers that bloom for a few weeks in April and May, and all the OTHERS - less known and difficult to find, but some spectacular plants that we should be growing.

(The combined RMC-NARGS & CCSS plant sale last weekend at Jeffco Fairgrounds was absolutely amazing for the breadth of plants offered. While many people contributed to its success Panayoti deserves special kudos for driving to specialty nurseries around the West collecting so many unusual plants for this sale - including some red phlox! See Phlox Liana at right.)

Since allergy season is here, Paula Ogilvie discusses the botany of pollen to help you understand what’s really getting up your nose. And Keith Funk answers more timely gardening questions.

Be sure to check out the Plant Sale and Garden Tour Page on our website. If you know of other sales or tours that should be listed here (it’s free) please let us know!

- Jane Shellenberger

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  Stake tall growing flowers that are prone to water and wind damage, like the ones you mentioned, should be staked while they're still short and before they need the support.  Once a plant has been allowed to blow over, it never looks quite right again.  At the garden center you’ll find many types of support structures available at a wide range of price points.  Being a penny pincher myself, I save small branch trimmings (2-3’ long) off my trees and shrubs during the year and stick them into the soil at angles creating an interwoven “cage” in May.  The plants quickly grow and hide the supports.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  Seed tender annuals like marigolds, zinnias, and cosmos directly into the garden by mid-May.  Most all annual transplants and bulbs can be set out by mid-May.  Impatiens, begonias, cannas and other warm loving annuals will prefer you wait until the soil is warm and night temperatures are above 50F. before planting.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  April and May are excellent months for controlling Japanese Beetle grubs that overwintered from last fall.  My personal choice is Phyllom Grub Gone granules.  It’s a bacterial product, very specific to scarab beetle grubs only.  This means the product will not harm beneficials like bees, butterflies, and ladybugs. It’s even approved for organic vegetable gardening. 

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

The bees are visiting all the cheerful blue chionodoxia flowers that have spread around in my beds. I’m also partial to early Jetfire daffodils that bloom at the same time. These are one of the most commonly available, inexpensive, smaller narcissus. They’re also sturdy. Mine were already blooming during the recent snowstorm  and they bounced right back. If only I’d planted the two together! 

Birds are also incredibly active right now since April is mating and nesting season. Unless you’ve been living under a rock you’re probably aware that lots of bird species are in trouble.  Climate change is affecting migratory patterns and artificial outdoor lighting messes with their navigation - big time.   Minimizing outdoor lights or putting them on motion sensors is one easy thing we can do.  Here’s a link to a story from our Spring 2023 print issue, “The Dark Garden”, telling you more. 

And then there are cats. First let me say that I have two and they do go outside during the day – but with clown collars! There are an estimated 43 million households in the US with at least one cat as a pet and I know that even well fed cats will catch birds given the chance. Add to this all the feral cats and you get the picture. Birds Be Safe (www.birdsbesafe.com), makes brightly colored collars that will make your cats look like silly clowns, but also allow birds to see and escape them. They're easy to put on and cats don't mind them at all.

Scientific studies from 5 countries show an average reduction of 60-87% of birds caught by cats wearing these collars. That’s huge! Don’t be fooled into thinking your adorable feline isn’t a killer.

Plant sale season is coming right up! One of the earliest, biggest, and best is the joint sale of the Colorado Cactus & Succulent Society (CCSS) and Rocky Mt Unit of the North American Rock Garden Society (RMCNARGS) at the Jeffco Fairgrounds April 27 & 28. You will find perennials and rock garden plants from local and regional growers that aren’t always available elsewhere, as well as wildly popular cactus and succulents for indoor hobbyists, and hardy, opuntia hybrids with astonishingly beautiful flowers.

Perennial field at Desert Canyon Farms in Cañon City

Last spring I made the trek to Desert Canyon Farms in Canyon City. They are only open to the public from mid April to mid June each year but the variety of plants they grow and sell (over 2300!) is amazing. Here’s a link to a story about owners Tammi & Chris Hartung.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Our feature this month focuses on growing with cold frames. Idelle Fisher, our web designer and an avid gardener - of vegetables and natives in particular, wrote it. She also writes a blog called Good Environmental News. As she points out in the piece, growing your own produce year ‘round is not only possible, but it reduces plastic consumption. (On that note, here’s a plug for Ridwell.com, a company that picks up your non-recyclable plastic and partners carefully with reputable companies like Trex, Byfusion and Hydroplox to turn it into usable building materials, keeping it out of landfills.)

Happy Spring!

~ Jane Shellenberger

By Idelle Fisher 

Cold Frames are basically a small, short version of a greenhouse – more or less a raised bed with sloped lids that trap solar heat to keep the bed warm. Sometimes called “Dutch Lights,” cold frames are a much less expensive alternative to greenhouses for growing in winter months and some say they are even more effective for growing winter greens or hardening off seedlings in the spring without added heat.

Cold Frames don’t take up any more room than a standard vegetable bed and you can build them easily yourself. You can use old skylights, windows, glass doors, etc, but polycarbonate plastic paneling is stronger and won’t shatter into your bed during a hailstorm.

In a Denver Urban Gardens class (dug.org) I learned about building cold-frames and about a great book, The Year-Round Vegetable Gardener: How to Grow Your Own Food 365 Days a Year, No Matter Where You Live, by Niki Jabbour. It’s packed with information including a handy diagram for building a cold frame. 

With my Dad’s woodworking expertise, tools, and his and my sister’s help, we built three cold frames a couple years ago, one for each of us. My dad’s and sister’s were smaller, using the scraps of plastic for “window panes.” (Dad’s idea… genius!) I purchased the 6mm polycarbonate sheets from coloradoplastics.com, which provide insulation and heavy-duty protection. I like the idea of recycling old windows or glass shower doors, but the thought of picking out broken glass from a hail-pummeled garden bed was a disincentive, though some with stronger/thicker glass may be better than others.

Auto-Vents are Crucial for Cold Frame Vegetable Health

Due to the shade of the house and a nearby evergreen tree I didn’t think 3-5 hours of sun would be enough, but it actually got quite hot in the cold frame on sunny winter days. In fact, if I didn’t open the lids, my cool weather-loving vegetables would cook as temps inside can easily and quickly climb past 100˚ F on warm sunny winter days. I bought a Univent brand vent from acfgreenhouses.com with a wax-powered piston that expands as it warms up, opening the cold frame, and closing it when it cools down – no electricity required. Cool-weather greens and veggies don’t like super hot temperatures, so venting is essential all winter long.

No Heat Needed!

It’s hard to believe, but with a little cover, cool-weather veggies thrive even during freezing temperatures. The added wind protection and warmer soil helps keep cold-weather veggies alive. They don’t grow fast in the short, cold days of winter, but with the cold frame lids closed they stay alive without added heat or insulation and then really start to take off in March and April.

Don’t forget to brush the snow off cold frames during the day so they can gain solar heat! I use a broom.

Watering

I water my cold frame during the spring, fall and winter months, but not that often. If we have a couple weeks of sunny, warmer weather I may water every one to two weeks, but when it’s cold I usually don’t water at all. I typically open both lids on nice sunny days in the winter when it’s over 45-50˚ F. Fresh air and breezes help cut down on pests and mold/dampening off. I check the soil and if it starts drying out, I water well with a watering can.

Mulch

Leaves and twigs help insulate seedlings and plants, so I add a light layer throughout my bed in the winter which also feeds the soil and the worms. You’ll discover that the leaves “disappear” as the worms pull them down and munch on them leaving fertilizing worm castings as thanks. Don’t smother your seedlings with mulch, however, they can’t grow if they’re completely covered.

Growing Your Own Food Year-Round Reduces Plastic & Pollution

One way we can cut back on plastic and emissions is to grow our own food. Just think about it; when you buy vegetables at the grocery store, plastic is often part of the equation. Organic lettuces, brussels sprouts, cabbage, carrots, and celery, for example, usually come packaged in plastic containers or bags. With my cold frame for winter growing, I can pick fresh lettuce, cabbage, spinach, green onions, parsley, cilantro with no trip to the store or plastic packaging required.

Treating Wood Cold Frame with Stains or Oils

We used naturally pest- and rot-resistant cedar wood for the cold frames. However, I wish I had treated mine sooner as the wood started to dry and crack after one winter season. My sister and Dad treated theirs, and they also disassemble and store them in the summer months, so their wood looks a little nicer for sure! My husband and I first used boiled linseed oil, but found it to be really stinky for a couple weeks - not ideal for our vegetables, wildlife or us.

So, this year I used Vermont Natural Coatings semi-transparent PolyWhey® Exterior Penetrating Wood Stain in the color: Lakeside Cedar. This company uses whey proteins instead of toxic ingredients found in traditional wood finish. (Their products also have ultra-low VOC levels, and are free from carcinogens, mutagens, and reprotoxins that are still found in many products claiming to have low or no VOCs.) Their penetrating wood stain goes on easily, has a thin consistency that doesn’t gunk up your brush, and it hardly smells at all compared to other stains. It provided a nice waterproof finish for the summer. I also talked to a local greenhouse builder who uses beeswax to protect wood greenhouse frames. Throughout history beeswax has been used as a natural wood treatment and waterproofing sealant. Beekeepers sometimes protect their wood hives with a combination of beeswax, walnut oil, and ground wood charcoal, heating it to mix, then cooling to apply. I’ll have to try that someday!

I’d Build a Taller Cold Frame Next Time

If I build another cold frame, I will make it taller. Mine is about 14″ high in the back, shorter in the front and when veggies get growing they touch the inside of the closed lid. Some extra height would also be nice for storing summer vegetable seedlings during the hardening off period in May/June. Another solution is to dig down to make extra headroom.

You can build a wood-frame cold frame out of lots of things and you don’t have to be a carpenter. If you have access to bricks or wood beams, you could build a simple back wall, and then drape plastic or a window to create a warm protected area.

Hoop Houses Have More Headroom

Hoop houses are also great for winter growing and though most have to be manually vented, they do provide more headroom for growing taller vegetables and storing seedlings and potted plants during cold spells or storms. I was lucky to get a hoop house from Earth Love Gardens in 2020, just as the pandemic was starting. This simple garden bed hoop house is made from metal conduit hoops and long-lasting Agtec Super Strength Clear Woven Greenhouse Film fabric (12mm) that should last 7+ years. Aaron designed the sidewalls to be held in place, with a large rolling panel that can be rolled up easily to vent the hoop house on sunny days. It’s very sturdy, withstands heavy snowstorms, hail, and wind very well, and it allows me to grow taller and potted plants in the winter. The greenhouse film is far superior to regular plastic for longevity and durability.

Native plants, peppers, and other veggies staying warm & protected in the hoop house in early stormy June last year

What can I plant in Colorado in April?

You can start lots of seeds directly in your cold frame bed or hoop house throughout the fall, winter and, of course, in early spring. April is a great time to sow seeds for lettuces, kale, spinach, mustard, cabbage, celery, onions and herbs like dill, cilantro and parsley. They will germinate and grow much more quickly than in uncovered outdoor garden beds (where row cover can help some, by increasing the temperature a few degrees) during our often freezing spring nights. Put Seedlings in Cold Frames or Hoop Houses in early Spring Hoop houses and cold frames are great for hardening off indoor-grown warm-season vegetable and flower seedlings, providing a protected space outside to acclimate them to outdoor weather until you transplant them into the garden. (If you have room indoors with bright light(s) you can keep these inside until nighttime temperatures are over 50-60˚- typically late May and early June in Denver.)

In April and early May, before moving some of my indoor-started seedlings out into my cold frame and hoop house I put out a “test subject” or two to see how they fare with cold nights. A spare basil plant is a good indicator since it wilts at the first chance of frost. I also test a pepper or tomato seedling for a few nights before moving more in. As long as it doesn’t drop well below freezing overnight, these hold up well, even with a few spring snowstorms. If a deep freeze is expected I bring them back into the house. I also start native plant seeds and keep them in my cold frame and hoop house over the winter to get them growing well before transplanting in spring.

Don’t forget to vent on sunny days, even if there is snow on the ground!

Another tip for Peppers and Tomatoes

Kept in the same small pot tomato or pepper seedlings will not grow nearly as fast as those that are potted up into larger pots as they grow. Especially with tomatoes, remove the bottom leaves and bury the stems, leaving about 2-3″ of the plant above ground. This allows for a deeper root system and more growing room which makes a huge difference for these plants.

Colorado Cold Frames & Greenhouses Group

If you love gardening, you should definitely add cold weather growing options like a cold frame or hoop house in your backyard. If you live in Colorado, join our Facebook group called Colorado Cold Frames & Greenhouses. I created the group to encourage sharing of photos, successes, and tips for winter growing. Lots of people have joined and it’s interesting to see everyone’s creations and methods. 

Happy Growing!

Written by Idelle Fisher, our web designer and an avid gardener - of vegetables and natives in particular. She also writes a blog called Good Environmental News.

A:  April and May are excellent months for controlling Japanese Beetle grubs that overwintered from last fall.  My personal choice is Phyllom Grub Gone granules.  It’s a bacterial product, very specific to scarab beetle grubs only.  This means the product will not harm beneficials like bees, butterflies, and ladybugs. It’s even approved for organic vegetable gardening. 

Safe for children, pets, and birds it’s proven to be every bit as effective as chemical applications.  Don’t worry if the granules get into your flower beds or shrub beds.  Grubs feed there as well.  Once applied, water it in thoroughly so it penetrates into the soil where the grubs live.  It doesn’t do any good sitting on top of the ground.   We all know Japanese Beetles can fly so controlling the grubs in your own yard will not prevent beetles from flying in from outside your property.  However, you can rest assured that the roots of your lawn and valuable landscape plants are safe from damage.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  My first application of plant food on my perennials is in early April.  I prefer a granular fertilizer with an analysis of 10-5-5 or something in that ballpark. that I can apply with a spreader and then water in thoroughly.  Any reputable brand of organic flower garden fertilizer will do.  Just follow label directions and repeat as often as recommended.  I feed monthly from April through September.  I’ve even used organic lawn food on my perennials in a pinch because I know organics are slow-release and won’t burn the foliage.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  Yes. There are several cold-tolerant annuals and perennials that will flower beautifully during our cool/cold spring weather. Violas, pansies, stock, wallflowers, snapdragons, brunnera, hellebores, creeping phlox and alyssum will easily tolerate light frost and even snow. If the plants are purchased from inside a warm greenhouse, be sure to gradually acclimate (harden off) your new babies to outdoor conditions before planting them. Garden centers often keep these durable plants in an outdoor sales area so they can be planted out right away.  

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  Remove spent flowers but keep all the foliage to rebuild the bulb’s strength to rebloom again next year.  Show them some love with an application of a general purpose flower garden food watered in thoroughly.  Once the foliage turns yellow it can be cut off at the soil surface.  DO NOT PULL the foliage off.  You run the risk of pulling the bulbs out or damaging the bulbs where the foliage attaches below ground, encouraging the bulbs to rot at the point of injury.  

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  Shrubs that flower before Memorial Day can be pruned immediately after flowering.  These shrubs include lilac, forsythia, mock orange, snowball bush, and flowering quince to name a few.  Pruning before  will remove this year’s flowers.  

Shrubs that flower after Memorial Day, like Rose of Sharon, Butterfly Bush and Blue Mist Spirea can be pruned as early as late winter.  You’ll want to finish pruning these shrubs by the time they start leafing out.

Keith Funk who is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

A:  That depends on the plant. Most perennials can be lifted and divided now, in early spring as you see new growth emerging. However, some perennials are prima donnas and will sulk after dividing. Peonies are a prime example. It may take them a couple of seasons to flower again. Others like daylilies, daisies, sedums, iceplant, goldenrod, and other summer bloomers probably won’t even notice they’ve been touched. When replanting your divisions be careful not to plant them any deeper than their original depth. Replanting too deeply is a surefire way to kill your treasures. Feel free to use a Sharpie to mark the stems at the soil line before digging them up, just to be on the safe side.

Summer blooming Hardy Geraniums can be divided in the spring.

Dividing Horseradish roots should be done when the leaves have been killed off by frost in the fall.

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Everbearing raspberries like Heritage are pruned differently. All the dark colored canes that produced fruit last year during the summer should be cut to the ground.  Keep all the new, lighter canes that grew last year. Once the old canes are removed, select the strongest remaining canes from last year and prune them to 4 feet tall.  Remove any thin, spindly growth at this time as well. 

Keith Funk is handling our Q & A this year. You may have visited one of Keith’s Denver area gardens on a tour or know him from his years at Echter’s and now at Nick’s Garden Center. For 30 years he’s been heard every Saturday morning from 7-9 with fellow Wise Guy, Jim Borland, on the Garden Wise Radio Show on Legends 810AM.

Plant Select Director Ross Shrigley says that it took years to bring this plant into their plant promotion program. He says that while it’s one of the best plants in the trade, it proved difficult to propagate so it didn’t meet one of Plant Select’s key criteria. But Gulley’s in Fort Collins finally “cracked the code” which boiled down to taking cuttings in the spring instead of fall. So this year EversilverTM creeping germander, (Teucrium ‘Harlequin’s Silver’) shown above, tops the list of Plant Select’s “New Plants for 2024.” The bloom period is relatively short – just two weeks in early June, but the pretty, silvery foliage persists year round. It thrives with little water in full sun and deer and rabbits don’t like it.

Others on this year’s list are:

Letitia flannel plant (verbascum ‘Letitia’), a uniformly globe-shapped perennial with masses of bright yellow flowers from May into July that attract pollinators, but not deer or rabbits. Noteworthy as a xeric plant that grows well in full sun or part shade, it won the Royal Horticultural Society’s Award of Garden Merit.

Sarada’s Greek mountain tea (Sideritis syriaca ‘P023S’) is a great replacement for tradtional Lamb’s Ears. It has the same velvety soft leaves but maintains a compact form without spreading and resists disease. Full sun, low water, and pale yellow blooms from late June to early July.

Crystal Frost™ Arizona cypress, (Hesperocyparis arizonica ‘Fandango’), is a native tree selected from a higher elevation along the Arizona/New Mexico border that can handle extreme temperatures and drought as well as winter temps to -20°F. This beautiful tree offeres excellent nesting sites for birds and shelter for small wildlife.

It seems fitting to wish you Happy Almost Spring on the cusp of the biggest snowstorm so far this winter. Maybe all this precipitation will satisfy my daughter temporarily. She moved back to the Front Range a year or so ago after eight years in Oregon where she could walk into a tall forest with miles of public trails just a few steps from her house in town. 

She’s been grousing about too much sun and the lack of trees here. I used to feel that way at first when I moved here eons ago, but no more.

On the Front Range we live in a semi-arid grassland steppe, not a woodland. Old photos show clearly that very few trees are native to the plains and those that are, grow in riparian areas. (The late Dr James Feucht, Horticulture Professor Emeritas at Colorado State University in Fort Collins, told me there were just three: willow, cottonwood, box elder.) Our urban trees were all planted and they have a lot to contend with: lack of humidity and regular precipitation, compacted, poor and high pH soils, improper planting and care, heat stress, rapid extreme temperature swings, pollution. All these stresses make them more vulnerable to insect pests and diseases. The tough trees that survive were, understandably, overplanted and this lack of diversity contributes to their vulnerability. The arrival of the Emerald Ash Borer in Colorado in 2013 underscored that fact.

I recently attended the annual Tree Diversity Conference, virtually this year. The brainchild of conference chair Sonia John, it brings together experts from around the country and the world to present their research in Denver and to explore the connections between species diversity, landscape design, and horticultural practices.

The high point for me was a presentation by Henrik Sjöman, Senior Researcher at the Swedish University of Agricultural Sciences & Scientific Curator at Gothenburg Botanical Garden.

He is co-author of The Essential Tree Selection Guide for climate resilience, carbon storage, species diversity, and other ecosystem benefits. (Long title but good description.) I’ve tried to summarize some of the conference discussions below.

But first, here’s a look at this year’s list of new plants from Plant Select.

~ Jane Shellenberger

p.s. Look for an article on season-extending cold frames by Idelle Fisher on our website soon. Idelle is our web designer and an avid gardener. Her blog is called Good Environmental News.

As with so many problems affecting the natural world, biodiversity is key, even though human domination of the planet is having the opposite effect. But that’s a larger story.

Here’s a summary of this year’s discussion of strategies, successes and solutions for creating a diverse urban environment.

1) Henrik Sjöman is Senior Researcher at the Swedish University of Agricultural Sciences & Scientific Curator at Gothenburg Botanical Garden. He stressed the importance of tree selection asking, What kind of plan does the plant (tree) have? How well prepared is it to take the punches of heat, drought, flooding, etc? The book he co-authored with Arit Anderson, The Essential Tree Selection Guide for climate resilience, carbon storage, species diversity, and other ecosystem benefits contains an A-Z list of over 550 trees with these benefits. (This is the same book mentioned by Kenton Seth in his piece, Trees in the Apocalypse in our Harvest Issue.

Sjöman said that we do our research when buying a car so why not be at least as thorough with tree selection? It’s not just about winter hardiness and autumn color. Genetics is important. There is different genetic material within the same species, as well as much variation in the wild, even in the same growing conditions. There is a new era of plant hunting to find natural growing habitats that match urban environments, and a lot of work to do as we are losing these habitats. For example, trees growing on the south side of a ridge have developed strategies to take the heat while the same trees on the north side like it cool and moist, and wouldn’t be good street trees. And some trees want to be alone while others grow in groups – the “social capacity” of trees. What makes trees able to manage or too fragile to cope? On plant hunting trips it’s important to meet with local farmers and foresters; they hold the knowledge. He also talked about the importance of creating teams, “working with the fungi people, the bug people, the soil people (the hidden half)” and with nurseries.

2) Nina Lauren Bassuk, professor and program leader of the Urban Horticulture Institute at Cornell University, gave a presentation on Evaluating Your Site to Increase Diversity in the Urban Forest. She showed slides from a list of smaller trees with attributes that might work in Colorado including ‘Winter King’ Hawthorn (C. viridis), Acer miyabei, Zelkova serrata ‘Wireless’ (for beneath powerlines), Golden Rain Tree (Koelrenteria paniculata), Lindens Tilia americana ‘Redmond’ and Tilia tormentosum, and many others.

The cooling shade of trees can be critical on a sunny day in cities where temperatures can reach 135° F. But different sides of the street vary considerably in the amount of sun and humidity.

Compaction and rubble from construction as well as sidewalks and salty streets impact soils that become as hard as brick. Tree roots often can’t make it out of their initial planting area. While we can select trees for many factors like heat, cold, insect & disease resistance, salt to some extent, we can’t choose trees for soil compaction so we need to do something about it. She said the “Scoop & Dump” method of soil remediation has worked well and showed before and after photos to illustrate the point.

Scoop & Dump

• Apply a layer of 6-8” of compost to compacted soil

• Use a backhoe bucket to dig down 18”

• Lift bucket with topsoil/compost mix 3 ft into the air

• Soil/compost mix is dropped onto the ground and smoothed

• Landscape plants are directly planted in the soil

• Surface mulch is added every year to replenish organic matter until there is canopy closure

2) Elizabeth Judd, Urban Forestry Planner for the City and County of Denver, presented results from a 2023 survey in which Denver residents were asked what they think about trees as part of an effort to create an Urban Forest Strategic Plan with citizen input. The top 5 answers (and their order) were a surprise to city foresters:

1. Beauty

2. Environmental Benefits (air quality, stormwater, etc)

3. Feel Better (mental health)

4. Wildlife

5. Property Value

Perhaps the main takeaway is that “trees need to be on the list for development project budgets,” Judd explained.