How and when to begin to fertilize seedlings | TribLIVE.com
Jessica Walliser, Columnist

How and when to begin to fertilize seedlings

Jessica Walliser
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Jessica Walliser for the Tribune-Review
When seedlings develop their first set of “true” leaves, it’s time to transplant them into larger containers. Fertilization begins soon after.

I started some broccoli, Brussels sprouts and parsley seeds under a set of grow lights a few weeks ago. I also plan to start some tomatoes and peppers soon. I’d like to know how and when to start fertilizing them. Right now the early crops are about an inch and a half tall. Everything looks good, but I want to make sure they’re healthy when I move them out to the garden.

Seed starting is a favorite hobby of many gardeners. It allows us to grow many more varieties than we can get at a nursery, plus it’s cost effective and fun. Using a lighting system is a smart way to keep the plants growing straight and strong.

If you used a potting soil formulated specifically for seed starting (as you should), there’s typically a small amount of fertilizer included in it to help your seedlings get off to a good start. There isn’t a lot of fertilizer added, however, since too much can burn young seedlings and their roots. As your plants grow, they quickly use up any fertilizer found in the seed-starting mix, and you’ll need to start feeding them a supplemental fertilizer.

Fertilization should begin soon after your seedlings form their first “true” leaves. The initial leaves that emerge from a seed are called the cotyledons. They’re rounded with smooth margins. The second set of leaves to emerge are the “true” leaves. They look very similar to the foliage of the mature plant. When the first set of “true” leaves has fully emerged, it’s time to move your seedlings to the next stage in their care.

First, when the “true” leaves arrive, it’s your signal to transplant the seedlings into larger containers or cell packs, using a standard potting mix that already contains a nutrient source. There are lots of different potting mixes out there you can use, but I prefer ones that contain naturally derived nutrients, rather than brands containing a chemical fertilizer.

Next, about two or three weeks after transplanting, it’s time to begin to fertilize the seedlings with a liquid organic fertilizer. Dilute the fertilizer to half the strength recommended on the bottle, and use it every two to three weeks. Choose a product formulated for use on seedlings (I use Espoma’s Grow), liquid kelp or fish emulsion.

Your strong, healthy seedlings will have to be hardened off before planting them out into the garden. This is an important step in the process to avoid burning the plants in the hot sun or freezing their tender foliage during cold nights.

For two weeks prior to moving the plants out into the garden, work on slowly acclimating the seedlings to outdoor conditions. Begin by placing them in a shady spot outdoors for just a few hours. Gradually leave them outside for longer periods of time and expose them to more sunlight until they are outside full-time. This hardening off process is extremely important to those young transplants and helps them gradually adjust to brighter light levels, wind and fluctuating outdoor temperatures.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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