Most gardeners understand seedlings need light to grow, but it’s surprisingly common to not know that many seeds need light to even start growing (sprouting), and some will only sprout in total darkness. If you get this need for or avoidance of light incorrect, you might have a tray of lots of good sprouts or, and this is annoying, a tray of seeds that just don’t appear.
A substance in seeds called phytochrome controls how they respond to light. Phytochrome comes in two types, and these respond to different colours within light. For some seeds, when they soak up red light (and there’s a lot of red in sunlight) the phytochrome becomes active and ‘tells’ the seed to begin to sprout. Other seeds do the reverse; they begin to sprout only when light is not present. Scientists who study plants divide seeds into three types based on this: those needing light (positively photoblastic), those needing dark (negatively photoblastic), and those which will sprout however much light is around (neutral).
Seeds That Need Light to Germinate
Seeds that need light to start growing are generally very small, almost as if they’re meant to sprout right on top of the earth or very close to it, not deep down in the soil. Lettuce is the one everyone knows best; if you plant lettuce seeds too far down, they often won’t grow at all as they can’t ‘see’ the light they require to get going. Dill, snapdragons, petunias, coleus, impatiens, begonias, chamomile, thyme and lots of different wildflower combinations are also among seeds which need light. With these seeds, when you’re beginning to grow them, you should lightly press them onto moist seed starting compost, and either not cover them at all or use the very finest covering of vermiculite. It just needs to be enough to keep the seed in contact with water but not to keep out the light.
Seeds That Require Darkness to Germinate
Bigger seeds that do best when starting to grow in the dark are usually ones that in the natural course of things, wind, rain or animals will push under the earth. Loads of the veggies we often grow are like this: tomatoes, peppers, peas, beans, corn, onions, and all the types of squash all will grow best if they’re completely under soil and in darkness while they’re starting to sprout. Calendula, nasturtiums, sweet peas and phlox need dark to begin growing too. Planting these seeds as deeply as is suggested, so generally two or three times the width of the seed, makes sure they are entirely protected from light while they are germinating.
Seeds That Are Indifferent to Light
Starting a lot of common vegetables is really easy because they’ll sprout whether they’re in the light or dark. Basil, cucumbers, zucchini, carrots, radishes, and most of the brassica family (that’s broccoli, cabbage, kale) don’t need light. You can just put those seeds down as deep as the packet says, and don’t worry about whether they’re going to be sunny or not. Soil temperature, how wet the soil is, and how old the seeds are are far more important for getting them to start growing, much more important than light.
Practical Tips for Getting Light Requirements Right
Good seed companies will tell you how deep to plant the seeds, and this considers whether they need light to get started. If the packet says to “surface sow” or “do not cover,” the seed needs light. But if it gives you a definite number for depth, the seed will do better in darkness. When the packet doesn’t say how deep to plant, the size of the seed is a good guide. Very tiny seeds, smaller than a grain of sand, almost always require light. Larger seeds, the ones you can easily pick up between your thumb and first finger, will practically always be happiest when covered with soil. If you’re not sure what to do, plant some seeds on the surface and some a little below, and that will quickly show you which way works best for when you plant them later.
Key Takeaway
Garden seeds are all different when it comes to how much light they need to start growing. Really tiny seeds – lettuce, petunias, dill for example – as a rule require light to sprout and you should just lay them on the surface of the soil, don’t bury them. Conversely, bigger seeds, tomatoes, peppers, beans, squash, will sprout most successfully in the dark, and when you put them at the correct depth in the soil. The easiest thing to do to find out if a specific seed needs light, dark, or doesn’t care either for germination is to look at the instructions for planting depth on the seed package.
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