by Thomas T Watson
A lot of what we do as gardeners amounts to little more than following instructions. Take planting a seed, for example. How deep do you need to plant it? Well, read the seed packet, of course, and you'll find out. Trust those instructions; they are the result of years of experiment and experience.
But why does it matter how deep, so long as the seed is covered up? It does matter, and the reason depends on the seed involved. Some, lettuce for example, require light for germination. Plant them deep enough that they receive no light, and you get no lettuce. Plants that need light for good germination tend to have small seeds, and seed size provides a clue to why light is needed. A tiny seed, germinating too far down, will run out of the food stored in it before reaching the surface. Only after the tiny leaves reach daylight can the seedling make its own food through the process of photosynthesis. If the seed detects light, it "knows" it is shallow enough to germinate successfully.
Other seeds (parsley, for example) need to be in the dark to germinate well. These are frequently the seeds of species that need a lot of moisture in order to germinate, and being down away from the soil surface makes sense for them. The soil surface will dry out more quickly than soil that is even an inch or two lower. Light tells these seeds they are too close to the top, where they are likely to dry out and die as the seedling tries to develop.
It all raises the mind-boggling idea that a seed can see. Well, not quite. Seeds perceive light, which is a slightly different thing. Seeds contain (among a great many other things) pigment molecules called phytochromes that react to light. The effect of the reaction depends on whether the seed needs light in order to germinate, or prefers darkness. In a lettuce seed the reaction triggers the germination process, but in parsley, the reaction inhibits germination. Think of phytochromes as light-activated on/off switches, and you get the idea.
There are, of course, seeds that do not respond to light one way or the other, and need only be deep enough that they remain moist in order to germinate. These matters and others are considered when planting advice is printed in books and on seed packets. Follow this advice faithfully, and you've taken a big step toward a successful garden.
But, of course, there are times when you won't want to plant exactly as the instructions say. Fortunately, the recommended planting depths have some margin for error built into them. Your gardening conditions will often change the planting depth to a certain degree. In a warm, dry spell, when the soil surface dries out quicker than normal, you might want to plant any given seed up to twice the recommended depth. This way you keep that seed, and the delicate seedling it will become, below the dry zone at the surface. More than twice the recommended depth is usually much too deep. If drought or heat are serious problems try shading the garden, or laying down a layer of loose mulch, something light enough that it won't impede the emergence of the seedlings. (Loose, shredded paper works well in this situation.) By the same token, early in the season, when the soil is still cold, the warmest soil will be nearest the surface. If you are pushing the season, try planting at half or two-thirds the regular depth to keep the seeds in this warmer zone. You will need to be extra careful to make sure these shallow plantings remain moist enough, and once again, a thin, light-weight mulch should do the trick.
Should you ever, for whatever strange reason, find yourself with seeds but no seed packet, you can estimate the planting depth for yourself. Put several of the seeds on a flat surface and get an idea of how high they sit; plant the seeds roughly three or four times this measure into the soil. Chances are good that, at such a planting depth, a fair number of the seeds will either be deep enough or shallow enough to do the trick. A useful trick, but frankly, I'd be more careful about saving seed packets, if I were you!
About the Author
Thomas Watson is graduate of the University of Arizona College of Agriculture, where he earned a BS in Agriculture, majoring in plant sciences. When he is not writing he is usually working in the garden, re-landscaping his Tucson home, studying botany and plant identification, or "out there" somewhere, studying the natural history of the Sonoran Desert. This is all, of course, based on the assumption that it isn't baseball season, in which case he'll get all of the above done before the next Diamondbacks game.