When Should I Start Feeding My Plant Seedlings and with What?

Sweet pea (Lathyrus odoratus) seedlings, Norfolk, UK, March
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Some gardeners think their seedlings will grow faster if they give them fertilizer. Those tender, tiny plants you start from seed indoors may look helpless, but they don't really need anything other than water, warmth and light, for their first few weeks. They are capable of feeding themselves, up to a point.

When Should You Start Feeding Your Plant Seedlings?

When your seedlings develop their first set of true leaves, it's time to give them some fertilizer.

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 actually leaves at all. They are called cotyledons or seed leaves. They are part of the seed or embryo of the plant. Cotyledons contain the remainder of the stored food reserves of the seed and keep the seed fed until the true leaves sprout and the plant can begin photosynthesis. Usually the cotyledons disappear shortly after the true leaves appear and begin photosynthesizing. At this point, they could use a little boost of fertilizer.

What Type of Fertilizer Should You use to Feed Seedlings?

Before you reach for the plant food, make sure you haven't used a potting soil that already had fertilizer in it. Some do, some don't. If the potting mix has fertilizer in it, you shouldn't need to add more.

For the future, since your seedlings can initially feed themselves, you don't need a potting mix with fertilizer for starting seed.

Using a mix without fertilizer is cheaper, but more importantly, you can control how much and what type of food they get.

Seedlings tend to need a fertilizer high in phosphorous, like a 1-2-1, N-P-K ratio.  A liquid or water soluble fertilizer will make is easiest and be the quickest way for the seedlings to access nutrients.

You have a choice of organic or synthetic fertilizer, so read the label carefully and make sure you get what you are looking for.

  • Synthetic Fertilizer: If you are using a synthetic fertilizer, feed your seedlings weekly, but it's wise to dilute the label's recommendation by at least half. Tender seedlings can easily be burned by too much fertilizer. Really young seedlings can get away with 1/4 of what the label recommends for full grown plants.
  • Organic Fertilizer: Luckily there are more and more liquid organic fertilizers available, although they can still be hard to locate. A mix of fish emulsion and kelp will also give your seedlings all the nutrients they need to get started. (Thankfully, they've figured out how to make fish emulsion a little less potent smelling.) As with synthetic fertilizer, you can give your seedlings a dose of organic food weekly. Unless it is labeled specifically for seedlings, dilute it by half the label's recommendation. It's better to give them a little food on a regular basis than to risk burning those tender, little roots with too much.
  • Another option is to mix a granular organic fertilizer into the potting soil. Many gardeners will do this when their seedlings are ready to be moved from their starter cell packs to larger pots. However granular fertilizer can take awhile to release nutrients and take effect, so adding it when you are starting your seeds is a better option. Try to add it to the lower layer of potting mix and not let it come in direct contact with the seed. Even organic fertilizers can burn, if you use too much.

    Knowing When Your Seedlings Have Had Enough

    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. At this point in the seedling's development, we are more interest in growing a healthy root system, than sending up a lot of green leaves. Each plant will react differently, but 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.