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How to Care for Plant Seedlings

The Spruce / Margot Cavin

When you start seeds indoors, the tender seedlings are dependent on you for all their needs.   This includes getting fed. Some gardeners think their seedlings will grow faster if they give them fertilizer right away. However, while those tiny plants may look helpless, they don’t need anything other than water, warmth, and light for their first few weeks. They are capable of feeding themselves up to a point. After that, it’s time to start feeding them, following a few standard guidelines.

When to Start Fertilizing Seedlings

When seedlings first poke out of the ground, they are still feeding off the food stored in the seed.   The first couple of leaves that form are not leaves at all. They are called cotyledons or seed leaves, which are part of the seed or embryo of the plant. Cotyledons contain the remainder of the stored food reserves of the seed, and they keep the seedling fed until the first true leaves sprout and the plant can begin photosynthesis.

Usually, the cotyledons disappear shortly after the first true leaves form and begin photosynthesizing. It is at this point that the seedling can use a little boost of fertilizer.

Before you reach for the plant food, make sure you haven’t used a potting mix that already contains fertilizer. Some do, and some don’t. If the mix has fertilizer, you shouldn’t need to add more. For the future, because seedlings can initially feed themselves, you don’t need to use a potting mix with fertilizer for starting seed. Using a mix without fertilizer is cheaper, and more importantly, you can control how much and what type of food your seedlings get.

Selecting Fertilizer

Seedlings tend to need a fertilizer that’s high in phosphorous. Phosphorus stimulates root development and is a component of photosynthesis. Look for a 1-2-1 N-P-K (nitrogen-phosphorus-potassium) ratio on the fertilizer label. A liquid or water-soluble fertilizer is typically the easiest and quickest way for the seedlings to access nutrients. You’ll also have a choice between organic and synthetic fertilizer, which often comes down to personal preference.

  • Synthetic fertilizer: If you are using synthetic fertilizer, feed your seedlings weekly. However, it’s often wise to dilute the label’s recommendation by at least half. Tender seedlings can be easily burned by too much fertilizer. Young seedlings commonly can get away with a quarter of what the label recommends for full-grown plants.
  • Organic fertilizer: There are several liquid organic fertilizers available, though they sometimes can be hard to locate. A mix of fish emulsion and kelp can also give your seedlings the nutrients they need to get started and reduces the risk of burning your seedlings. As with synthetic fertilizer, give your seedlings a dose of organic food weekly. Unless the product is labeled specifically for seedlings, dilute it by at least half the recommended dose. It’s better to give your seedlings a little food regularly than to risk burning those tender roots with too much fertilizer at once.
  • Another option: Mix a granular organic fertilizer into the potting soil. Many gardeners do this when their seedlings are ready to be moved from their starter containers to larger pots. However, granular fertilizer can take a while to release nutrients and impact the plants, so adding it when you are starting your seeds is often a better option. Try to add it to the lower layer of potting mix, and don’t let it come in direct contact with the seeds. Even organic fertilizers can burn if you use too much.

Knowing When Seedlings Have Had Enough Food

How much to feed seedlings will take some experimentation. Keep an eye on how well your seedlings are filling out. Too much fertilizer can cause a flush of tender, lanky growth, which is not what you want. Ease back on the fertilizer if this is the case.   At this point in a seedling’s development, you should be more interested in growing a healthy root system than sending up a lot of green leaves.

Moreover, each plant—even those of the same species—will react a little differently to fertilizer. But in time you should get a feel for how much food it takes to keep your seedlings robust while they build up the strength to be moved outdoors into the garden.

New plant seedlings can feed themselves up until their first true leaves appear. Here are tips on how, when, and what to feed your seedlings.

How and when to begin to fertilize seedlings

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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 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.

TribLIVE’s Daily and Weekly email newsletters deliver the news you want and information you need, right to your inbox.

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