Sowing Seeds

Don’t Forget to Fertilize Your Seedlings

’At the beginning of April, I’m gradually starting my seedlings. In mid-March, I planted some verbena bonariensis. Since I don’t have many seedlings to start at the same time, it allows me to gather my material and to buy what I need and ease myself into it.‘At the beginning of April, I’m gradually starting my seedlings. In mid-March, I planted some verbena bonariensis. Since I don’t have many seedlings to start at the same time, it allows me to gather my material and to buy what I need and ease myself into it.

Photo : Mathieu Hodgson

At the beginning of April, I started som kale, parsley, dill and of course, tomatoes, the queen of the vegetable garden. It’s getting crowded in here! But it’s not over yet! This week, I will start some lettuce seedlings that will be transplanted to the garden in early May, for my first harvest. The last ones to come, indoors, will be cucumbers and nasturtiums, in early May.

So far, I’ve only seen a small head of vervain pop out of the potting soil, but, before long, seedlings will be popping out all over the place, just like in a whack-a-mole game (don’t worry, I won’t hit my seedlings with a hammer!). We don’t have much space in our home for seedlings. Under my desk is a shelf with two seedling trays. One is covered with a clear plastic dome where I keep my pots before germination. The other is not covered and I use it to transfer seedlings that have germinated. The whole thing is overhung with a full-spectrum LED light. Eventually, all those seedlings will get hungry! How to feed them?

The Ill-Fertilized Gardener

I’ll tell you a secret. Last year, I completely forgot to fertilize my seedlings. I didn’t even realize it because my garden did very well, despite the usual few failures ( as with every year, some plants do better than others).

Don’t get me wrong, I’m not suggesting you stop using fertilizers on your seedlings. This year, I won’t forget to do it, I swear. However, I do think we need to have a good understanding of why and how to fertilize so that we can do it in a sensible way.

Semis de tomate
Photo : jag2020

When to Start Fertilizing Your Seedlings?

I’ve heard several versions of the same story, but the general consensus is that fertilization should begin when your seedling has 4 to 6 true leaves. By true leaves, I am excluding the cotyledons which are usually the first two leaves of the plant and look different from the other leaves. In truth, the cotyledons are already inside the seed before germination and unfold when the conditions are right.

Why wait until the plant has 4 to 6 leaves? It’s simple! Those who eat, for example, pumpkin or sunflower seeds know how full of energy they are. A seed is therefore composed of a small embryonic plant and enormous energy reserves to allow the seedlings to grow until they have roots to feed themselves and leaves to perform photosynthesis. No leaves, no roots, no fertilizer!

It is also important to note that many potting soils are lightly fertilized. Check the packaging of your favorite potting soil. This might explain why my seedlings last year did well despite the lack of fertilizer. Another reason to wait until our seedlings are developed before fertilizing.

Engrais pour semis
Photo : Mathieu Hodgson

So What Do We Use as Fertilizer?

When you’re a laidback gardener, you work as little as possible. I rarely use water-soluble fertilizers because I have to add them regularly and it’s too much work. So I use a slow-release, all-purpose organic fertilizer that will release its minerals over the course of an entire season. But for seedlings, I make an exception. I want them to get nutrients immediately, not at the end of the year! So I use a water-soluble organic fertilizer that I add with each watering. However, you can use a quarter or half of the manufacturer’s recommended dose.

I don’t pay too much attention to the numbers on fertilizer labels that give the levels of nitrogen (N), phosphorus (P) and potassium (K), the major elements plants need to live. Plants will take what they need. That’s why I use all-purpose fertilizer. But if I had a fertilizer for annual plants on hand, I wouldn’t throw it in the garbage. It might as well serve a purpose.

Not Too High

But, be careful with fertilizers with very high numbers (like 30-30-30 ), as they may damage your seedlings. If you notice leaves with brown tips, stop or slow down your fertilization. It is possible that the fertilizer has built up in the soil and is now too concentrated.

Engrais bionik

For years, my father used liquid seaweed fertilizers, such as Bionik Liquid Seaweed 0.36-0-0.22. But there is a new product available on the market that I will try this year and that contains not only seaweed, but also insect frass: Bionik All Purpose Fertilizer 1-0.5-0.3. This fertilizer, certified organic by Québec Vrai is also a product of Québec. Its main ingredient, insect frass, is a residue from insect breeding containing their droppings and molts. Studies have shown that frass has the ability to increase plant growth and yield. In the garden, the chitin contained in this product from insect molts will feed the microscopic fauna of your soil, which un turn will help prevent diseases and pest infestations. Available soon in a garden center near you!

Compost in Your Seedling Mix?

Sometimes it’s suggested to mix compost with your seedling mix. Caution should be taken with this practice. While I love compost and believe it’s essential to the success of your vegetable garden, I suggest that you leave compost where it belongs—outside. 

Household composts that are too young or poorly prepared may contain pathogens due to the lack of oxygen in the composting process. However, conventional commercial composts (often composed of peat and manure) have normally reached a high enough temperature over a long enough period of time to destroy pathogens, such as those that cause damping-off. Also, compost that is not sufficiently decomposed may contain compounds that are toxic to seedlings. 

Don’t add too much compost, 5 to 10%, as it can affect the porosity of the soil. Seedling potting soils are very light. This allows good circulation of oxygen in the soil, which is necessary for the roots to breathe. Some composts, or too much compost, can weigh down your soil. Be aware that there are some risks to using home compost in a seedling mix. Also, keep in mind that compost works over a longer period of time, feeding the soil and the microorganisms that benefit the plants in it rather than feeding the plants directly.

Mathieu manages the and websites. He is also a garden designer for a landscaping company in Montreal, Canada. Although he loves contributing to the blog, he prefers fishing.

1 comment on “Don’t Forget to Fertilize Your Seedlings

  1. Margaret Coates

    Hello Mathieu. I just found your blog when I searched for the difference between geranium and pelargonium and saw that you had responded to many of the questions and comments. I recently moved from Vancouver Island, where I harvested vegetables all winter, to Zone 3, just east of Edmonton. I have a lot to learn and relearn about gardening and I’m going to be reading your blogs to see what you have to say. So enjoy your fishing but don’t forget about us gardeners out here in the hinterland.

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