| "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 " " 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
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