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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.

Should indoor seedlings get fertilizer?

Fertilizer can be added to seed trays along with every other watering.

How soon should tomato seedlings be fed? I’ve started some under lights inside and have some granular 10-10-10. Can that be mixed with water to bottom feed? If so how much water to how much granular?

A: Tomatoes and most other vegetable and flower seedlings should get fertilizer once they develop their second set of leaves. The first leaves are the “seed leaves,” and plants at that stage don’t need fertilizer yet.

Baby plants should get only a dilute fertilizer – about one-quarter to one-half strength of the dose recommended on the bottle or bag.

Liquid fertilizer is easy to mix, and so is dry fertilizer formulated for dissolving in water, such as the popular Miracle-Gro or brands like Peters or Jack’s.

Your granular 10-10-10 has filler that won’t dissolve. The nitrogen in it will, but it’s really not intended or formulated for seedlings. That’s why you won’t see seedling rates listed on the bag.

I haven’t even seen any “official” suggested rates for dissolving 10-10-10 for seedlings, although some gardeners say they’ve done what you’re thinking at a rate of 1 to 2 tablespoons of 10-10-10 per gallon of water with success.

Personally, I’d stick with the 10-10-10 outside and buy a balanced liquid or water-soluble fertilizer for your seedlings.

I use organic fertilizer on my seedlings and look for products that are slightly higher in phosphorus (the middle number in the formula), such as a breakdown of 1-2-1. Even something close to balanced is OK (3-3-3, for example).

Organic products are less likely to cause salt buildups in the soil. You’re also less likely to overdo it with organic fertilizers.

Garden centers usually sell water-soluble fertilizers formulated for seedlings and sometimes even specifically for tomatoes. Catalogs also carry targeted organic fertilizers like this or at least organic products such as fish emulsion and kelp or seaweed extract, including Gardens Alive, Gardeners Supply and W. Atlee Burpee Co.

Bottom-watering is the way to go. I dissolve my fertilizer in water and add it to the tray for the seedlings to suck up as opposed to pouring the mix onto the soil or over top of the plants.

I add the dilute fertilizer every other watering. I typically add water to the trays about twice a week and so fertilizer weekly.

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Should indoor seedlings get fertilizer? Fertilizer can be added to seed trays along with every other watering. How soon should tomato seedlings be fed? I’ve started some under lights inside