Discovering the science trapped within the seed
FRANKLIN, MASS. — It is mid-April, and the last frost date for my area is about six weeks away. It's the perfect time to start marigolds, cosmos, and zinnias indoors, and lettuce, spinach, and radishes outdoors.
I also have several packets of perennial seeds, since I have a large garden and purchasing enough mature plants would be expensive. In exchange for the price of the seeds and a bit of patience, I can have that hillside of glowing lupines and an avalanche of Alaska daisies.
A seed is a kit, complete with built-in instructions, sensors, rations, and sometimes weapons necessary to launch a new life. A basic understanding of a seed's structure can greatly improve the chance that those you choose to plant will germinate and thrive.
The tough outer skin of the seed protects the plant embryo until signals from the environment tell it to start growing. This skin helps keep enough moisture in the seed to keep it alive, but not so much that it will be damaged if it freezes. This works so well for some arctic lupines that 10,000-year-old seeds excavated from permafrost germinated.
Some seed coats are so tough that they have to be damaged or removed through a process called scarification.
Lupine, nasturtium, moon flower, and morning glory seeds should be gently nicked with a nail file. This abrades the seed allowing water to enter and germination to begin. Check packet instructions to see if this treatment is necessary.
Plants native to cool temperate climates, including popular perennial species such as purple coneflower, phlox, asters, and columbine, have a mechanism that tells them when it is safe to begin growing. In the wild, these species drop their seeds in the autumn. The seeds contain a chemical that prevents them from germinating. This chemical lock is broken down by near-freezing temperatures. Unless they are exposed to cold, these species cannot grow. This is called stratification.
You can duplicate this chill in a refrigerator. The seed packet usually tells you if that species needs to be chilled and for how long. Put the seeds in a plastic bag with a slightly moistened paper towel or a little moistened planting mix. Label the bag, including the date chilling began and the date seeds can be planted.
Light is another environmental cue. Exposure to light signals the seed that it is near the soil surface, so it will have enough energy to sustain itself until its leaves begin to function. In fact, the smaller the seed, the more likely it is to need light to trigger germination. Generally, you should not bury a seed deeper than its own length.
I scattered three species of poppy seed on the top of a new bed several weeks ago without covering them, and they will be fine. Yesterday I planted my lettuces about a quarter of an inch down, since they will respond to the diffuse light that penetrates the soil surface.
Seeds also respond to temperature. Peas and fava beans prefer the cool soils of early spring, but carrots, beets, and bush beans will rot if planted in soil below 70 degrees F. The temperature of most homes is fine for starting seeds, although warmer is better and seeds will germinate more rapidly.
Germination depends on the right amount of water. Too much will keep the plant from absorbing oxygen and can promote "damping off," a fungi which destroys the stem at the soil surface. Too little water, and the seedling cannot develop enough leaf and root tissue.
Whether planting indoors or out, prepare the soil so it is light and fluffy and water drains from it quickly. In the garden, add generous amounts of organic material: peat moss, well-rotted compost, or commercial potting soil and loosen the soil down at least a foot.
For indoor planting, use a fine soilless mix. I sift a standard commercial mix through a 1/4-inch wire mesh to remove the larger chunks, then blend five parts of mix with one part of vermiculite. This combination stays moist without becoming soggy or compacted, and is light enough to allow oxygen to reach the roots.
Next, if the seeds are large enough to handle easily, like beans, peppers, and tomatoes, soak them overnight in warm water. They will germinate more rapidly, and you will not have to keep the soil as damp for the first few days. When sowing outdoors, plant them at the recommended depth and water them gently. During hot and dry weather, cover the bed with moistened burlap, which you should remove when the first seedlings appear.
Before sowing indoors, moisten the planting mix until it feels like a well-wrung sponge. Fill your containers well, but don't pack the mix down. Plant seeds in the moist mix at the appropriate depth and then place clear plastic wrap over the container, but do not seal it entirely. Remove the wrap when the first plants emerge, and water gently with a mister.
Marigolds and radishes will reward your care by appearing in a few days. Cosmos, lettuce, and most perennials will take a week or longer. But the satisfaction of spotting those first bright leaves is worth the wait.
*Anne Hollerbach, a long-time gardener, has a PhD in history, specializing in the history of life sciences. She does her sowing and gathering in Franklin, Mass.