Basic Seed Starting & Baby Plant Feeding

Q: I'll be starting basil and tomatoes under some LED fixtures that I can adjust to keep the lights just above the plants. I'll use a timer to provide the appropriate amount of light each day (which I still need to research). My question is about feeding. I've heard you say that potting soil has almost no nutrients. So how should I feed my young plants? Once they're outside, I'll use my own compost (made from fall leaves), but I don't want to use it inside the house. Is there some kind of granular feed you would recommend?

---Tyler in Harleysville PA

A: I have to thank Tyler for his timeliness; this is the perfect time of year to deliver a little primer on seed starting; and I'll begin by suggesting that those lights be on for 16 hours a day, and then off to give the baby plants eight hours of darkness every cycle.

Side note: His use of LEDs is clearly the Next Big Thing in indoor growing, but I don't know a lot about them—and I already have several sets of four foot long fluorescent tube fixtures that work very well for me. However, the basic principle is the same: you want to keep the baby plants very close to your lights to insure that they get enough lumens, and that means you need lights that don't get too hot. Old school incandescent bulbs would cook the baby plants in much the same way that the Easy Bake ovens of my childhood used 60 watt incandescent bulbs to bake cupcakes.

Timing: start your seeds two months before you intend to put the plants in the ground. If you typically plant on May 1, your start date is March 1st. May 15th = March 15th; and in colder climes, April Fool's for June 1st—so you don't get fooled by a late frost.

Tyler already seems to know not to use garden soil to start his seeds. It's the worst. Using a light, loose soil-free mix allows for the maximum root growth and good drainage that produces healthy plants; and beginning the season with healthy vigorous starts gives you your best shot at long-term success. Trying to start seeds in clay soil and the infamous "sunny windowsill" always delivers weak, puny, leggy plants. You've got two strikes on you before the season even starts!

There are many excellent seed-starting systems on the market, but I use old plastic nursery 'six packs' that I've collected over the years. They're designed to hold moisture, drain well, and provide the kind of depth that allows plants to develop deep roots. (Plus re-using the containers gets rid of any potential guilt I might have about using plastic. By the time I throw an old six pack away, it's in shreds!)

Note: whatever you use to start your seeds, follow one rule: No egg cartons! They're not deep enough and you're not in kindergarten anymore!

Fill the containers with a light, loose soil-free mix and sit them in water for a few hours until the mix becomes totally saturated. (You may have to put a dish or something on top to hold them down in the water in the beginning.) Then sow two seeds on top of the wet medium in each cell. (Don't sow and then water; it can wash the seeds all over the place.)

Cover the seeds with a little more mix, cover the containers with something like Saran Wrap (or put the lid on if it's a little plastic greenhouse) and put them in a warm but not hot spot. Mid 70s F. is ideal. If you're planning on doing this every season, heating mats are a great investment. They're set to the perfect temperature to get the seeds sprouted quickly without cooking them. All my seeds are started on mats.

Place your freshly-sown and now covered containers in a low tray that holds water, and add water whenever the tray gets dry. Check them every day, and take the wrapping or cover off as soon as you see the very first sprouts. Immediately stop using any bottom heat and get your plants under bright light—with no more than an inch between the lights and your baby plants.

Don't worry about the seeds that haven't sprouted yet; any viable ones will pop up soon. Keep the uncovered plants close to the lights and cut back on the water.

Now: when and how do we feed?

Most of the high-quality bagged organic/natural potting and seed starting soils I've seen and used already contain small amounts of natural nutrients like worm castings and compost that feed the baby plants perfectly for the first few weeks, making life easier and the young plants healthier. And seeds also contain a lot of their own 'starter food', so even if you don't have a little bit of natural food in your seed-starting mix, don't add any food until the plants are a few inches high and have at least four true leaves.

(Note: Avoid seed-starting and potting soils that contain chemical fertilizer, which is what you're most likely to find at mass merchants. Take a pass on these and keep looking; it's better to start a week later than to feed your baby plants nasty stuff in utero. Same goes for those 'water-holding crystals'; I've never seen any evidence that they work (and they creep me out).

Then before we feed, we 'thin'. This is the hardest chore emotionally in seed-starting, but it has to be done. Use a small pair of scissors to snip off the weakest looking sprouts until you only have one plant per cell. Choose the shortest, stockiest, best-colored specimens for survival; snip off the taller, more spindly ones. And be sure to snip the losers out—don't yank them out or you risk killing the other plant!

Then get a bag of high quality compost or worm castings and make 'tea'. Fill an old sock with the compost or castings, let it sit in a quart of water for a few hours and then use that 'tea' to water and feed your plants. (You can use your own compost if it's completely finished, but most gardeners don't have finished compost this early in the season.)

Depending on the size of your original seed-starting cells, you might also want to pot your plants up into larger containers, which is never a bad idea if you have enough room under your lights.

This is a great time to mix some worm castings or compost in with the additional potting mix you'll be using. (Notice how gently we're feeding early on?)

Note: For tomatoes, add all the new soil on top, burying part of the stem and keeping the roots down at the very bottom. But with all other plants, add the new stuff at the bottom and maintain your original soil line. (Read a couple of our tomato articles to learn why.

Hardening off: As planting time approaches, take your babies outside for a few hours a day for several days, and then leave them out overnight as soon as nighttime temps remain in the 50s. When the calendar says it should be OK and the ten day weather forecast confirms that it is OK (nights in the 50s), you can plant them outside. Don't rush the season; tropical plants (like peppers, tomatoes, eggplant, melons, cukes….) do poorly if they're exposed to nighttime chill early in life.

Then wait a week to let them get settled before you actually 'feed' them. That food can be a 'mulch' of two inches of high-quality compost or one inch of worm castings or a balanced non-chemical fertilizer. If you're using a granulated fertilizer, spread the directed amount on the surface of the soil and then cover it with some compost or soil to activate it to release its nutrients faster.

For more detailed info on all of these steps, read our earlier articles on seed starting under the letter 'S" in our archives.