Seeds:The Next Generation
By Paula Ogilvie:
From the dust like seeds of orchids to the 95-pound double coco de mer coconut, seeds contain the embryos for growing new vegetables, flowers or woody plants. Composed of an embryo, stored food, and a protective outer seed coat, seeds require three conditions to germinate and start growth: the right temperature, water, and appropriate soil conditions.
The seed coat protects the dormant embryo until conditions are optimal for growth. Impervious to water, coats are usually a dullish brown blending in with the soil to discourage predation. Seeds only contain about 2% water. A single warm, wet day in winter isn’t optimal for growth to begin, nor are the hot dry days of late summer. Once the correct temperature and sufficient rainfall arrives, dormancy-enhancing hormones are washed away. A tiny pore on seeds allows water uptake (also called imbibation). While too small to see, the pore is generally located next to a scare-like area, easily seen on large seeds where the seed was attached in the fruit.
In nature, staggered germination allows plants to survive over the years in case growing conditions become unfavorable after growth has started.
Some seeds require scarification, a process where seeds coats are scratched before allowing water to enter. This can occur when seeds roll along the ground, pass through an animal’s digestive tract or when home gardeners use a tool to scratch it.
Larger seeds have more food reserves and may be able to survive in less favorable environments. In Colorado’s mountains Lodgepole Pine seeds are only released after a fire. Windborne seeds often lack both a seed coat and nutritive layer. Others, like beans, have coats so thin that a gentle rub will remove them. Seeds of commercially popular vegetables have often been selected to germinate when planted – requiring just soil and water. Too much water can cause a seed to rot. Like all living things the embryo needs oxygen for growth to occur and development to continue.
In nature, staggered germination allows plants to survive over the years in case growing conditions become unfavorable after growth has started. For example, a late freeze will kill a new seedling but others will then start to germinate. Some embryos require a longer time to develop within the seed before they can germinate. Some plants, even the parent plants, can produce chemicals in the soil that prevent seedlings from growing nearby to prevent competition for resources.
A developing embryo first produces a root which takes up water and nutrients while anchoring the plant in place. Inside larger seeds the embryo may contain a rich layer of nutrients, the endosperm, and the first seed leaves called cotyledons. Small seeds with less food reserves are produced in greater numbers to ensure that some will survive.
The endosperm layer is rich in carbohydrates, oils, amino acids and other nutrients. Cotyledons or seed leaves distinguish flowering plants in one of two groups. Embryos with one seed leaf are called monocotyledons, those with two seed leaves are dicotyledons. When young plants first emerge the seed leaves help provide nutrition until the true leaves form. Once depleted they shrivel and die, and the true leaves take over, turning green and producing all the food the plant needs through photosynthesis.
A general rule is plant a seed to a depth about the same as its length. If in doubt, better to plant a bit shallower than too deep since the embryo might deplete its stored nutrients before reaching sunlight. Often tiny seeds like evening primrose and lettuce are not buried but scattered on the ground since light is required for germination. Others like chives, garlic and fennel are inhibited by light. It is best to follow the directions on the seed packet for required planting depth. For collected seeds research using seed catalogs, nursery staff or the internet for guidance.
Plants in dry environments like the desert southwest often produce smaller seeds that can stay dormant for years until there is a favorable rainy season. 10,100 year-old seeds have been found inside frozen lemmings - and they germinated! Seeds 1,400 years old found in a peat bog also grew. To increase survival, many plants have evolved to produce seeds that germinate at different times and/or have a few different methods of dispersal. Weeds excel at producing numerous seeds, some of which grow immediately while others remain dormant for years. There is a saying, “One year’s seeds, seven years weeds”, which explains why weeds may show up for years even in a well weeded plot.
Seed packets contain more seeds than necessary for a home garden, especially a small one. You can save seeds from your garden in a paper bag or envelop in a cool, dark, dry location and plant them the following year. Many can be saved for up to 2 or 3 years. Or more so be sure to label and date. Cucumber and melon seeds can be saved for 6 years. To test old seeds place in a dish of water for about 15 minutes. If they float, toss as they’re probably no longer viable. Also there are seed visibility charts online.
If seeds not are planted by May or June place them in a bowl of tepid water for a few hours or overnight. Drain and rise off the seeds and plant. Discard the water as it will contain growth inhibitors. This technique will jump start the germination process and works well with larger seeds like cucumber, peas, beans, and corn. They will need to be planted immediately after soaking or will die.
Watching seeds emerge from the ground or a pot indoors can be fascinating. Notice the short-lived seed leaves, then the true leaves. Dig up one young plant and note how much longer the root is than the stem. Beans are the easiest for observing germination. Soak overnight, then keep them covered with a moist paper towel. Every few days check for root development and then split open a seed to observe the developing embryo.
Seeds for home gardeners are not GMO. However, purchased seeds that are pink or red have been chemically treated to prevent mold. This should not affect the plant. (Years ago, seeds of Heavenly Blue morning glories had a blue coating of arsenic to prevent people using them to get high. This practice has stopped but if encountered it will be noted on the seed packet.)
The old farmers rhyme to “plant one for the mouse, one for the crow, one for the worm, and one to grow” is sound advice.
Paula Ogilvie is a former instructor of botany and biology in Denver.