Supporting Trees in Colorado
By Mikl Brawner
Trees do a lot for us so we shouldn’t forget to give them some support. When I look at the treeless 10-acre lot next to our nursery, or see an old photo of the CU campus with bare land around Old Main at CU Boulder, I remember why we can’t take trees for granted in Colorado. Trees really have it hard here in the Steppes, but there are things we can do to help them survive and thrive.
To begin, we must choose varieties that like Colorado conditions. The CSU website has a valuable downloadable resource to help us, the Front Range Tree Recommendation List. 250 trees were evaluated by local experts and the trees are rated A-D for their success. Just because you see stunning photos or have fond memories of Pin Oaks, Red Maples, and Bradford Pears doesn’t mean they will thrive in our alkaline soils, violent temperature swings, and the worst fireblight in the country.
For example: never plant an apple or pear that is not resistant to fireblight. A Radiant Crab is rated “A”, a Royalty Crab “D” because of fireblight. Our alkaline soils make certain nutrients, like iron, less available, resulting in weakened trees of some varieties.
It’s also important where you buy a tree. Think long term. Trees are not well cared for in Big Box stores and a stressed tree is a poor investment. Some balled-and-burlap trees come from Oregon or other dissimilar environments. B&B trees do better if they are locally grown – and if they are not too big. People want to start with a big tree, but the bigger the trunk diameter, the more roots that are left in the ground when they are dug. A tree with a 2” trunk diameter will need less water to establish and will be less stressed than a 4” diameter tree, and it will start growing more quickly. Container-grown trees have the advantage of complete root systems. They are smaller to start, but because their roots have not been damaged, they need less watering and begin to grow immediately.
With our average annual rainfall between 12” and 20” we need to water most trees. Lawn irrigation systems are not designed for trees, which want deeper and less frequent waterings. In fact, most trees would be a lot happier outside a lawn. Jack Phillips, principal of New Tree School, has said, “Trees did not evolve in landscapes dominated by lawn…Progressive tree care requires a movement away from green sterility and toward a wilderness in soil.”
What Jack means is that not only does grass drink a tree’s water and starve it for oxygen, chemical lawn care starves it for organic matter and kills off the soil life that are a tree’s allies. Trees need less water in spring when we have precipitation and twice as much in July when it is so hot and dry. But remember: soggy soils have little space for air and trees need oxygen in the root zone as much as they need water.
Because of our typically dry autumns, sunny winters, and winter warm spells it is very helpful to trees to winter-water, especially evergreens and trees planted in the current year. Water early on a sunny day to give time for the water to soak in before night-time freezing and long enough to penetrate 6”-8”. No need to water if the ground is frozen. Remember: Colorado is in a semi-arid region, we are a mile closer to the sun, and we lack the winter cloud cover of woodland regions. Our soils are not the rich glacier deposits of the Midwest where I grew up. Clay is okay but it takes more than water to grow strong trees. There are exceptions: native oaks, Russian Hawthorn, Boxelder. But, from a tree’s point of view, unamended Colorado soils are deficient in organic matter and nitrogen. There are those who disagree with me, but I will stand my ground based on 35 years in the tree care business.
I do not believe in using chemical fertilizers which are made from natural gas, lack micronutrients, and are water soluble. They push fast soft growth that susceptible to fungal diseases and sucking insects. I do believe in planting a tree with 30% compost and a little organic fertilizer. And I think periodic feeding with an organic fertilizer and minerals is good, especially if you also are giving the tree some mycorrhizae and mulching over the root zone. I favor this approach because I believe it is necessary to make up for the lack of natural pile-up of leaves and branches that feed the soil on forest floors. Yes, there are already mycorrhizae in our poor soils, but experience shows that trees establish faster, have fewer disease problems, and need less water when we give them a good mycorrhizae supplement or compost tea. And yes, trees will grow without fertilizer, but with good nutrition they will grow faster and be stronger and more able to cope with Colorado’s harsh environmental conditions. Two good times to fertilize are early spring and early fall, but early fall, when trees are storing nutrients in their roots and stems, is best. This is where the energy will come from if they have to make a new set of leaves after a late spring freeze or ward off an insect or disease.
According to research done by Ralph Zentz, a Fort Collins city forester, 70%-90% of all tree problems are abiotic. That means few tree problems are caused by insects and diseases; most are caused by environmental and cultural conditions. This is good news since it means we can generally support our trees without spraying (especially with something toxic).
So choose a tree that likes Colorado, plant it in the sun or part shade if it prefers, water it deeply and infrequently including once a month in the winter, prune it correctly, and give it organic nutrition so that its soil allies will prosper. This will give it the vitality to breathe carbon from the atmosphere and build a structure that withstands wind and snow, while providing shade, fruit, beauty, and habitat.
Mikl & Eve Brawner co-own Harlequin’s Gardens in Boulder, which specializes in organic veggie starts and herbs, natives, sustainable roses, xeriscape, unusual perennials, and products to build healthy soils.
