When you sprout flower or vegetable seeds indoors, the first set of leaves that you’ll see are called cotyledons or seedling leaves. These leaves keep the new baby plant fed until it can begin photosynthesis. You should never fertilize a seedling until you see a second set of leaves form. They are called the first “true” leaves of the plant.
Some of my flower sprouts are finally showing their first set of true leaves. Because sterile soil mixes contain little or no nutrients, I begin fertilizing the plants as soon as those true leaves appear. I use a fertilizer specially designed for seedlings. I fertilize them about once every two weeks. If you do it too much you can burn the plant. The leaves will turn brown and fall off.
I water my plants from the bottom the entire time I’m tending to them inside. Once I bring them bring them outside in the spring to be hardened off I switch to watering from the top. Watering from the bottom ensures you won’t knock over any weaker seedlings. Because the roots have to travel downward to reach the water, your plants will grow stronger.
I fill a plastic planting tray with warm water (or warm water with fertilizer if it’s a fertilizing day) and place the individual seedling paks in it until they start to feel heavy when I lift them. That usually takes about 2 to 3 minutes. I try to take them out of the water before the top of the soil is wet, especially with flowers like zinnias that are prone to damping-off disease.
The first time I fertilize the seedlings is always a thrill. It would be fun to set up a time-lapse camera sometime to capture the action. Within 12 hours the plants get beefier, straighter and darker green in color. I’ve had seedlings shoot up a quarter-inch or more overnight after receiving their very first dose of fertilizer.
Even though some of my seedlings are ready for their first feeding, I may hold off on fertilizing for another week or two. Spring is being dragged along kicking and screaming and I’m starting to worry that our planting season could start quite late this year.