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A hand holding a beaker of seaweed extract to feed to seedlings, shown in the background in trays in a greenhouse
All Things Garden,  Beginner Basics,  Indoor Gardening

How & Why to Feed Seedlings Seaweed Extract

If your new baby seedlings are a few weeks old now, it may be about time to feed them for the first time! In best practice, seeds are sowed in very mild, light, fluffy seedling starting soil mix, which is generally pretty devoid of nutrients. That is fine (for now) because tiny seedlings do not need or like fertilizer in the first couple weeks after sprouting. It can actually harm them, or “burn” the seed and prevent germination!

On the other hand, as they begin to grow, that fluffy seedling mix quickly becomes too light and won’t be nutritious enough to keep them happy for very long. Feeding your seedlings with seaweed extract can help solve that! It is gentle, sustainable, and effective.

Let me show you how we feed our seedlings seaweed extract to help keep them healthy and strong!

It is very simple, and will make for a pretty quick post! At the end, you’ll find a demonstration video.

When do I start feeding my seedlings?

Seeds are pretty amazing little things. The seed itself contains all the food and nutrients that the little plant they produce needs for those first few weeks after sprouting. But as they start to mature, they’re also going to start to get hungry. Like any good baby should, they will get very cranky if you don’t feed them when they want it. Signs of distress include yellowing leaves, stunted growth, and even disease or death.

The best time to start feeding your seedlings is before they begin to get cranky. It is a lot better to keep them satiated then wait until they’re in trouble, and try to correct the damage. This is sort of a Goldilocks and The Three Bears kind of story. Not too early, not too late, not too much. We want “just right”.

Wait until after first couple sets of “true leaves” appear, and then start to feed them very lightly. About 3 to 4 weeks after germinating is a good target, about the time you’d want to start to thin them also.

What are true leaves? When a seed germinates, the first set of little leaves that emerge (often heart-shaped, and often looking exactly alike between dozens of varieties of veggies) are not the true leaves. These are the cotyledon leaves – their embryonic leaves. The two leaves that come after the cotyledon are their “true” leaves. Those leaves will more closely resemble what the mature leaves of the plant will look like.

This image shows three photos of very small seedlings. They all have their embryonic leaves, also called cotyledon, and are just starting to grow their first set of true leaves. Many of them all look the same at this stage.
These seedlings are TOO SMALL to be fed any fertilizer yet! The image in the top right show the heart-shaped cotyledon on a bunch of broccoli, bok choy, kale, and mustard greens. The lower right are tomato sprouts, and the on the left is a tomatillo. Wait another week or two, until the true leaves become larger and another set starts to appear.


If you’re going to plant your seedlings outside or pot them up within a few weeks after germination, it may not be completely necessary to feed them in their starting container. When they are planted outside in a bed of rich soil, or into a larger container with fresh soil and compost, they’re going to be essentially “fed” in that process. Yet if you are like us, and keep seedlings in containers for two or three months before planting them outside, they’ll definitely want a few rounds of food during that time.

A bench in a greenhouse is filled with homegrown seedlings, all several inches tall now and starting to look very full and healthy. Shown are tomatoes, peppers, various greens like kale or collards, herbs, and flowers. There are long grow lights hung above them.
Seedlings about this size would be ready for their first feeding. These are all about 3 weeks old.

What should I feed my seedlings?

Many gardeners, us included, like to use a dilute organic seaweed extract. It is nice and mild, making it very difficult to shock or harm your seedlings unless you really overdo it.

Seaweed extract helps the plants grow bigger and develop stronger root systems. Both of these contribute to overall improved plant health and immunity. Just like people, a plant with a strong immune system has a stronger ability to fight off disease, pests, or rebound from stress. Seaweed extract is loaded with over 70 beneficial vitamins, minerals, micronutrients, and enzymes! It contains magnesium, potassium, zinc, iron and nitrogen – to name just a few.

Cold-pressed kelp is commonly used to make seaweed extract. The harvest and cultivation of kelp is widely recognized as sustainable and environmentally-friendly! This is an excellent (and often overlooked) multi-purpose fertilizer; one that can be used for much more than seedling care! When we don’t have time to make a batch of compost tea, we water our garden beds or house plants with it too. Additionally, seaweed extract can be used to make foliar sprays. The plants can then absorb all that good stuff straight through their leaves.

This the seaweed extract that we love and use.

A hand holding a one gallon jug of seedling seaweed extract, in a greenhouse full of seedlings.
The seaweed extract we use! All those seedlings were fed with it last week, and are clearly happy as can be!

From the book “Seaweed Sustainability” – Academic Press:

“Seaweeds grow in abundance in the oceans, many of which are edible and safe for human consumption. They have been documented to contain many of the essential nutrients such as omega-3 fatty acids, amino acids, vitamins, minerals, and bioactive compounds.

For many years, seaweeds have also been cultivated and utilized directly as food for humans or as feed to produce food for human consumption (e.g. fertilizer). Since seaweeds grow in many climatic conditions globally, their cultivation has minimal impact on the environment. Seaweeds are increasingly recognized as a sustainable food source with the potential to play a major role in providing food security worldwide.”

Kritika Mahadevan, Chapter 13 – Seaweeds: a sustainable food source

A underwater image of a kelp forest. Long green strands of kelp grow upward toward the light blue water surface, and fish swim between them.
A kelp forest off the coast of Southern California. Photo courtesy of Ocean Safari Scuba

Kelp is such amazing stuff, that in addition to feeding it to our plants, we take some for ourselves too! No, not this liquid fertilizer… but in the form oral supplements. Algae is the only plant-based, vegan, or vegetarian source that contains all the most beneficial and essential forms of omega 3-fatty acids that are usually lacking in other plant sources. If you’re curious, read all about that here.

Another option people use for fertilizing seedlings is liquid fish emulsion. We don’t personally use this, so I won’t speak on its behalf.

How do I feed my seedlings seaweed extract?

Mix the seaweed extract with water according to the instructions on the bottle. If possible, use de-chlorinated water on seedlings. We use captured rainwater. If you allow chlorinated water to sit out, like in a bucket for example, the chlorine will dissipate in a day or two. If your city uses chloramines instead of chlorine to disinfect their water, it won’t burn off. When we can’t use our rain water, another option is to hook up this basic RV carbon filter to a hose – and that takes care of it.

Some types of seaweed extract have varying instructions for different types of plants or stages of growth. Look for instructions clearly intended for seedlings, or as a soil drench. We use about an ounce per one gallon of water, maybe just a touch over that sometimes.  

Personally, I like to mix it inside this one-gallon watering can, especially for watering seedlings or working in the greenhouse. It is easier to handle than a larger 2 gallon can, and I really love the long, curved, skinnier spout. The design makes it very convenient for watering from below, as described next.

Wait a few days after the last time you watered, until the plants are getting a little thirsty and are due for another routine watering. Now, feed the seedlings the dilute seaweed extract mixture in place of their regular water. To accomplish this, we prefer to water from below.

A water can is feeding seedlings seaweed extract by pouring the liquid into the bottom tray that seedling containers are sitting in. This is in a greenhouse on a wood shelf.
Watering from below, into the trays. A note about our seedling trays: we prefer to use these heavy-duty seedling trays. They’re incredibly durable, will not crack, and can even hold bricks without bending! In addition to their longevity and strength, they’re perfect for watering from below. We still have some older flimsy 10×20 trays that aren’t totally “broken” and fairly functional, but this year I noticed most of them have developed little pinholes in the corners! This means all the liquid leaks out when practicing watering from below. And…. I figured this out the hard way, using a tray inside!

Feeding seedlings seaweed extract from below:

If you aren’t familiar with the concept of “watering from below”, it is exactly what it sounds like. It’s accomplished by pouring water (or in this case, seaweed solution) into the tray the seedling containers are sitting in. The soil will draw up moisture from the bottom, soaking up as much as it needs until the soil is evenly saturated.

Simply pour enough seaweed solution to evenly fill the bottom of the tray (with the containers still sitting inside of it) to about a half an inch deep. It is important that your trays are sitting level to ensure all the seedlings are getting a similar amount of seaweed solution. Otherwise, the liquid may pool on one side and leave the other thirsty.

After adding the seaweed solution to the tray, wait about an hour to see what happens. Did they already suck it all up, but seem a bit dry still? If so, you may need to add a little bit more. Alternatively, is the soil totally saturated, but a lot of liquid remains in the tray?

Allowing seedlings to sit in soggy conditions is not ideal. They breathe through their roots and do not want to drown. Therefore, we try to remove any leftover standing liquid from the trays within a few hours. Or, at least by the next day if we get busy. You can do this by either very gently tipping it out (if possible), or using a large garden syringe thing to suck it out. Yes… it is easiest if you don’t have a bunch of excess, so I suggest to go lighter at first and add more if needed.

How often to feed seedlings seaweed extract?

You can generally feed seedlings seaweed extract every two to three weeks, depending on the brand. Again, read those instructions! After feeding, you should see an immediate boost in growth.


It is as simple as that. If you follow these steps to feed your seedlings, they will thank you and feed you right back!

Check out this video to see just how quick and easy it really is.

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Do you need more seed starting and seedling care tips? Check out this post all about starting seeds indoors! Furthermore, here is another one that covers how to thin seedlings when the time is right.

In all, I hope you found this interesting and informative! Let me know if you have any questions.


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