The Science of Seedlings

In foggy coastal Humboldt, you have to baby your babies

(April 2, 2009) Last month I offered up some ideas for getting seed started indoors. It’s not too late; summer vegetables like tomatoes and squash really shouldn’t go in the ground until mid-May at the earliest. If you live near the coast like I do, it is entirely likely that temperatures won’t reach tomato-friendly levels until June. So you’ve got some time.

But let’s assume that you already have some seeds going indoors like I do, and the plants are getting larger, and your little indoor nursery is starting to get a little jungle-like. Here’s what’s next:

Pot it up. All those little seedlings nestled in their seed trays need to move up to larger pots. If you’re like me, you probably have an embarrassingly large collection of plastic pots in the garage, so clean those with a mixture of hot water and white vinegar to kill diseases or insects and reuse them for your seedlings. I prefer 4-inch pots at this stage. If you have to buy them, most garden centers sell them for 10 or 20 cents a pot.

Get great dirt. This is really the time to splurge on high-quality potting soil. Get good, rich, organic soil so that your seedlings will stay healthy and grow quickly. This is not a good time to reuse old potting soil, which might harbor disease or being depleted of nutrients. You probably don’t need to add any fertilizer right away; a good potting soil will have all the nutrients your plants need to get going. So move everything up to a larger pot, making sure to keep them labeled so you’ll know what you’re planting when the time comes.

Raise your lights. Now that your seedlings are taller, they’re going to need a little more room under those grow lights. Some plants naturally have a more compact, low-growing habit, while others are going to shoot up and get pretty tall over the next few weeks. So you’ll probably want to organize your plants according to type, keeping some underneath a low-hanging set of lights, and putting the rest under a second set of lights that will have to be adjusted constantly as the plants get taller.

Pinch back blossoms. This is not the time for a plant to bloom. If your flowers are getting ready to burst into bud, explain politely that they’re getting ahead of themselves and pinch those buds off. You want your plants putting their energy into developing healthy root systems and the kind of branching structure that will allow them to bloom on multiple stems later in the season.

Feed — a little. It’s easy to burn tender seedlings by overfeeding them when they’re young. Get a liquid organic fertilizer and use it at about half strength when you water. Alternate that with plain water. You want your plants to be healthy, but not stressed. Once they’re in the ground, the roots will have the freedom to travel to get the soil they like best, but while they’re in pots, take it easy on them.

Keep the heat going. The weather is getting warmer outside, and maybe it’s getting warmer indoors where your seedlings are growing. But if you started them on heating mats, keep those mats going. The extra warmth makes them dry out quicker, which means you’re going to have to water more often, but that supplemental heat source will do more to get your plants ready for the summer garden than anything else you can do.

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