When To Fertilize Seedlings For Best Growth

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Are you new to gardening and wonder when to fertilize seedlings?

The best time to fertilize seedlings is after they have grown two sets of leaves, but there are factors, such as soil condition, that determine how much fertilizer a plant requires.

Follow the guide below where I explain the ins and outs of seedling fertilization to help you provide the nutrients they need so your gardening efforts pay off with healthy mature plants!

Should I Fertilize Seedlings?

Yes, it would be best if you fertilized seedlings so that they have easy access to nutrients. 

When you provide ample food directly to the root system, the plant doesn’t have to expend energy stretching its roots in search of nutrients. Instead, the plant can use that energy for faster growth of the stems and foliage along with healthy root expansion.

Most seed starting soil mixes are either sterile or low-nutrient because seeds have energy naturally stored inside the casing, so the plant can grow whenever conditions are right.

Once seeds sprout, the root will quickly draw up any nutrients in the growing medium, and if you don’t fertilize or transplant it into richer soil, the plant will lose vigor and may eventually die.

When To Start Fertilizing Seedlings

You will know when to start fertilizing seedlings by watching the growth of the leaves. Once a seedling gets two to three sets of “true” leaves, it’s time to begin a fertilization routine.

The first set of leaves you see on a seedling are the cotyledons, which form from the seed embryo and grow so the plant can begin photosynthesis to create energy. 

seedling first leaves sprouting

The cotyledons are easy to identify because they are oval or round and have smooth edges. 

The leaves that form after the cotyledons are a plant’s real leaves, and they will look different than the first leaves. 

The real leaves on a seedling will look like small versions of the leaves on a mature plant, typically more pointy and with uneven edges.

When you spot two or three sets of real leaves on a seedling, it’s time to consider your fertilizing options.

How old should seedlings be before using nutrients?

As a general guideline, you should expect it to take about three or four weeks for a seedling to grow two sets of real leaves. 

So, once you see your seeds sprout, it will be around a month before you’ll need to start fertilizing if your growing medium includes a low level of nutrients. 

On the other hand, if you use a sterile growing medium, you should start fertilizing about a week after your seeds sprout using the schedule I discuss in the section below.

If the growing medium is very porous, like coconut coir, or the pots are too small, you’ll need to move the seedlings into your outdoor garden or larger pots with a standard potting mix.

If you’re not ready to transplant the seedlings outside and the pots or trays have a good potting mix and are large enough to support more growth, you can fertilize them just as they are.

How To Fertilize Seedlings

The best way to fertilize seedlings is with a liquid organic fertilizer. 

Fish emulsion or kelp fertilizers are especially ideal for seedlings. These types of fertilizers provide the best balance of nutrients and trace elements, especially phosphorus and nitrogen, seedlings require to grow into healthy mature plants.

Avoid using granule fertilizers for seedlings.

Granular fertilizers may take too long to break down with regular watering, which will leave your seedlings without the nutrients they need for robust growth.

Granular fertilizers can also end up on the stems or leaves of your seedlings and cause burns that could kill the plant.

Liquid Fertilizer Ratio

Too much of any fertilizer on plants will build up salt around their newly forming roots, choking them off. Use caution when dosing your plants the first few times by using a light dose.

Do not use any fertilizer full-strength on seedlings.

For sprouts growing in sterile soil, dilute a liquid fertilizer to one-quarter strength and dose the sprouts every week, with plain water in between. Once the sprouts grow their two true sets of leaves, increase to one-half strength fertilizing solution, and decrease dosage to once every three to four weeks.

For seedlings with real leaves grown in low-nutrient soil, always dilute the liquid to half the strength of the recommendation on the label or only use a product made specifically for seedlings. Dose the plants every three weeks until you transplant them outside, then reduce to once a month.

In Summary

Knowing when to fertilize seedlings is just as important as knowing how. Fertilizing too soon or too late will both be detrimental to your plant’s overall growth.

I hope you use the information above, so your seedlings get the best start in life with all the crucial nutrients they need to grow strong and beautiful!