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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,  Seed Starting

How to Feed Seedlings with Seaweed Extract Fertilizer

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. Fertilizing seedlings with seaweed extract can help solve that! It is gentle, sustainable, and effective.

Let me show you how we fertilize our seedlings seaweed extract fertilizer 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 to fertilize 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 fertilizing 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 fertilize my seedlings with?

Many gardeners, us included, like to use a dilute organic seaweed extract fertilizer. 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.
Edit: We used to use this seaweed extract (at the time of writing this post), but the formula recently changed and it no longer says OMRI listed for organic gardening. I’m not sure what’s up, but we since switched to this organic seaweed extract instead.

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.

Click here to check out our YouTube channel. If you like what you see, please subscribe!

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.


  • Chris

    Hi Deanna, thanks for all the love you’ve poured into this site. I recommend it to everyone I know.

    My question is some of my seedlings are getting pink leaves. I went back to the seeding posts and I didn’t read anything.

    Can you tell my why they are Turing punk and how do I fix it?

    Thanks so much!!

  • Tyson

    Kinda off topic here, but the bottom watering section got me thinking about how bummed I was this year with flimsy watering trays and inserts that only last a single season before cracking and ending up in a landfill. Any recommendations on something more durable?

    • DeannaCat

      Hey! Not off topic at all! This is something we experienced early on as well, not only flimsy trays cracking, but sometimes developing tiny pin-holes in the corners of the ones that still looked okay – leading to them leaking everywhere and not properly under-watering. We upgraded to these durable bad boys and couldn’t be happier. They last forever and literally can carry bricks without bending. I hope that helps!

        • DeannaCat

          Hi Kama – We have a few different articles that discuss the way we feed our plants throughout the season. First, we typically amend the raised beds before planting (see that article here) and top them off with compost as mulch, which gives them a good bit of food for the season. Then, we also make aerated compost tea once every month or two. I hope that helps!

  • Kelly Nickell

    Hello Deanna! I started my seedlings about a month ago in 1″ cell pots. It seemed once they germinated they stopped growing so I moved some into 4″ pots. Again they seem to be growing very slow. Most peppers and tomatoes are just getting their true leaves. I did my first round of seaweed extract today. I am in MN and plan to plant more seedlings by May 15th. I just feel like they are growing very slowing and wont be ready in time! Do you have any advice for this issue? Thanks!

    • Jacklyn

      Do you use the seaweed extract after you have transplanted them into your garden bed as a feed throughout the growing season?

      • DeannaCat

        Hi Jacklyn – it depends! We usually amend our beds with other fertilizers and compost before transplanting out a whole big round of new plants, or water with mycorrhizae and/or aloe vera, so we’ll skip the seaweed then. But sometimes we do want to give them an extra boost a few weeks after planting, or if we transplant something to a spot that didn’t get the other treatments. Throughout the growing season, we rely on compost tea more than seaweed extract – but use seaweed if we don’t have time to make compost tea. I hope that helps!

  • Rakel

    Thanks for this helpful information. I hope one day you can do a blog post on feeding and maintaining potted fruit trees. I recently got a multi-graphed apple tree and im keeping it potted for a while until I determine the best spot to plant it in my yard. I follow you on IG and I would love to know your regimen for your espaliered apple tree. 😊

  • Liz M

    Thanks for this post! I was wondering if you were going to do a post about the aloe “juice” you make for your transplanted plants? I read about it on one of your IG posts. I couldn’t remember how to make it so I just blended up some aloe and water. I ended up putting it on after we planted and the transplants weren’t looking so good. I didn’t know if it would work, since it just sat on top of the soil. It worked great! I really think it saved some of our transplants that we waited too long on. Mahalo for all of the info that you share here and on IG!

    • DeannaCat

      You did good! That’s pretty much the jist of it! We blend chunks with de-chlorinated water and use it for transplant shock, or just a good nutrient boost. Yes I do a post about it in the near future! The key is to use it within a short period, no more than 20 minutes after blending, because it can oxidize, ferment, and start reducing in beneficial properties pretty quickly. Good work!

  • Teresa G

    Thanks for the product recommendations! It saves me so much time. So far extremely happy with the tray, dome, and light products. Just ordered the seaweed extract. I’m really tempted to buy the greenhouse but I have to remind myself not until I get back into gardening and have my layout finalized.

    • DeannaCat

      Oh good! I am so glad to hear it! If you get a greenhouse, go larger than you think you want! If you can fit it and afford it, of course! We wish we sized up for sure.

  • jennifer fuller

    My seedlings are just coming up in my square foot gardening box, what is the best way to water using the Seaweed fertilizer?

    • DeannaCat

      Hi Jennifer, I would wait until they’re a bit older than “just coming up” as I described in the post, but when the time is right, you can just mix it in a watering can (per the instructions on the bottle) and shower it over them when it is time to water. It is okay if it gets on their leaves. I hope that helps!

  • Lacey Daniels

    Woohoo! Excited to give my seedlings a boost with some of this stuff. Do you fertilize your plants once they are planted outside as well, throughout the summer? I plan to amend my containers with compost before planting them out and am concerned I’ll over feed them if I also fertilize with seaweed. Thoughts?

    • DeannaCat

      Yes we feed our plants while they’re growing about every 2 months or so, but most often with compost tea. That is, in addition to amending the best with some mild meal fertilizers and compost a couple times a year too. When we don’t have time to make a batch of compost tea, we will use the seaweed extract instead. I wouldn’t worry about “overfeeding” between compost and occasional seaweed. Both are very mild! It’s more when you get into blood meal, bone meal, and things that nature that it can get risky!

  • Jenelle

    i know your talking about seaweed fertilizer here… i need to finish up my fish fertilizer before i invest in this sustainable plant food!. Can i do this same thing with concentrated fish fertilizer? Dilute it in one gallon, then add it to my spray bottle. is it okay to spray my seedlings? i currently (mostly) just spray the soil around the seedling, not the seedling itself. Thanks! Keep up the good work- the blog is beautiful!

    • DeannaCat

      Sure. Just follow the instructions on your bottle for the amount to dilute for seedlings. Did you read the seed starting 101 post, re: the benefits of watering from below? If you like spraying from above, that’s totally coo! We have found it much more effective, quick, easy, and good for the plants to water (or fertilize) from below 🙂

      • Sarah Jo

        Hi, I was wondering where you got your greenhouse?
        I am in the market to buy one and feel overwhelmed by the choices. I live in Sacramento.
        Thank you!
        Sarah Jo

        • DeannaCat

          Hi Sarah Jo – Believe it or not, we got it on Amazon! And after years of use, we couldn’t be happier with it. If you check out this garden supply list, you’ll find our exact greenhouse along with the solar powered fans and auto vent arm we added. I hope this helps!

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