Seedling to Adulthood – Secrets for Growing Mature Plants From Seed

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Robert Pavlis

There is a lot of information about germinating seeds, but what happens after germination? How do you take care of the seedlings to get them to a mature size? When do you transplant seedlings? How much should you fertilize them?  How do you harden off seedlings before they go outside?

In this blog post I will walk you through the whole process I use to get seedlings to adulthood.

Seedling to Adulthood - Secrets to Growing Mature Plants From Seed
Seedling to Adulthood – Secrets to Growing Mature Plants From Seed

Germinating Seeds

I’ve created several blogs and videos about starting plants from seed, so in this post I will assume you already have some seedlings. If you want to know more about germinating seeds, see the following resources.

Best Method For Starting Seeds Indoors

Plant Seeds – Getting Started

YouTube Videos on Seed Germination

Different Types of Seedlings

You might be wondering which type of seedlings I am talking about in this post. These general rules work for 99.99% of seedlings, including houseplants, annuals, perennials, trees and shrubs. The exception might be some seeds that are planted in water instead of soil.

The seed could be planted directly in pots, or with my Baggy Method. If you Winter Sow you don’t have to fertilize as much or harden them off.

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Watering Seedlings

The seed has germinated and you see some green sprouts. You’re thinking, boy are those tiny, they must need constant watering, but you would be wrong.

The initial growth you see comes mostly from the food and water in the seed. A newly germinated seed only needs enough water to keep the soil moist. Water only when needed. When the soil surface starts to get dry, water. For the first couple of weeks it is best to water from the bottom by placing pots in a tray of water. After that they can be watered from above.

Preventing Disease

Young seedlings can get a number of fungal diseases. The best way to prevent this is not to over water and to keep a fan going 24/7. The fan helps things dry out, which makes the environment less attractive to fungal spores.

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Fertilizing Seedlings

True leaf on a tomato seedling
True leaf on a tomato seedling

Seeds store enough food for the early root and first leaves to develop. Seedlings do not need nutrients until they start making the next set of leaves – the true leaves. However, that does not mean you need to start fertilizing at this point.

The point at which you start fertilizing depends very much on the media you are using to start the seeds. Most people use a soilless media made from peat moss or coco coir. If the material was packaged for plants it probably has some fertilizer already added.

Promix is a common starting point and it has enough fertilizer for the first month or so. Sunshine, a similar brand, has less fertilizer and may need some added sooner. It is likely that you don’t know how much your mix contains unless you made your own. In that case start fertilizing once the first true leaves are about half formed.

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Which kind of fertilizer should you use? I like to use a soluble synthetic fertilizer that contains all of the micro nutrients. You can also use an organic fertilizer, but they rarely tell you the concentration of all the nutrients which means you might be missing some.

When you mix the fertilizer, use about 20% of what it tells you to use on the label, and use it with every second watering. Seedlings have smaller root systems, and are generally growing in low light conditions. They don’t need as much fertilizer as larger plants growing in higher light.

If you see any brown tips on the seedling, cut back on the fertilizer. It could indicate an accumulation of fertilizer salts.

Thinning Seedlings or Pricking Out Seedlings

If you planted one seed per container, you can skip this step. If you planted more than one in a container you have a couple of options.

Option 1. If you only put a few seeds in a pot, you can decide to remove all but one, and keep it growing in the same pot. This is a good option if you only want a couple of plants from a seed sowing, and you don’t want to disturb the seedling. Rather than pulling out the excess, cut them off with scissors, as soon as you know one of them will survive.

Video showing you how to thin seedlings: https://youtu.be/b-NOhVSSwi4

Option 2. A more likely scenario is that you planted numerous seeds in the same pot and that you want to keep several or many of them. In this case you want to “prick them out”.

Once the seedlings start to make their first true leaf, it is time to thin them. Take a pencil, tweezer or some other pointy device and gently disturb the soil around a single seedling. Try to pry it out with as little damage to the roots as possible, while holding the new leaf. Don’t touch the stem. Now plant this one seedling in its own pot using the same kind of media.

I actually like to take out a bigger clump, lay it on the table, and gently tease the seedlings apart. Then pot each one up singly. I think this does less damage to the seedlings, but it really depends on how large the root system is once you do this.

If you have more seedlings than you want, you can also pot them up in small clumps, wait a week or so to see which ones survive, and then cut off the excess with scissors as explained in Option 1. This does less damage to the root system.

After moving the seedlings, water them in well without fertilizer. You can use fertilizer on the next watering.

Transplanting Seedlings to Pots

At some point the seedling is large enough to be transplanted to either a larger pot or into the ground, and I’ll deal with these two events separately.

When should you transplant seedlings? When the root system tells you it’s time.

If you notice roots growing out of the drainage holes it is time to transplant into a bigger pot. From time to time, knock out a seedling from it’s pot and have a look at the roots. If they are starting to get root bound, it’s time to repot.

To knock out a seedling, place your hand over the pot so that the stem of the seedling is between the middle two fingers and your palm is resting on the pot. Turn the pot over and give it tap. Then pull the pot straight up to reveal the roots. When done, put the pot back on the plant and turn it right side up again. If done gently this will do little or no damage to root ball.

When growing roots reach the edge of the pot, they can’t grow any further and so they follow the contour of the pot. A plant is root bound when the roots start going around the inside of the pot, which is not good for the plant.

There is a common myth that says you should always move the seedling to a pot that is one size larger. This simply is not true. It is better for the plant to go right into a larger pot. The reason commercial nurseries go up by one size is that it conserves space, water and fertilizer. Provided your growing area is large enough, these are not really concerns for most home owners. If growing under lights space may be an issue.

Moving a seedling to a very large pot won’t hurt the plant, but it may be a waste of potting media. For annual plants that will be going into the garden soon, a 4″ or 6″ pot is lots. For most perennials, trees and shrubs, a 6″ pot works well. I like using the 1/2 gal square pots because they pack nicely together, but I also use a lot of round pots.

For the first transplant, use the same seedling media and plant them at the same height as they were in the seedling pot. Firm the soil gently around the roots and water right away to settle the media. Treat them just like a seedling.

How to Harden Off Seedlings

When your seedlings are ready to go outside, you have to “harden them off”, which is a process that gets them used to being outside. This process should not be started until outdoor night temperatures are consistently above 5C or 40F. If temperatures unexpectedly fall below this, you will have to bring them back inside for the night.

Harden seedlings off on the north side of the house, photo source: cristina.sanvito
Harden seedlings off on the north side of the house, photo source: cristina.sanvito

Two things harm seedlings that have been growing inside; wind and sun.

Wind is less of a problem, but small seedlings that are not used to wind can be harmed or even broken in half. The wind also dries out leaves thereby dedicating them to the point where they get white and crumbly.

Too much sun will burn foliage.

I like to place my seedlings right up agaisnt the north side of the house with some shelter from wind. A couple of days later I move them a few feet away from the house. This increases the light and wind. A few days later I move them even farther away from the house. Over a period of 7-10 days, they go from inside conditions to full outside conditions to harden off.

Shade loving plants are also treated this way, but they never get all day sun. At most give them half-day sun.

Once hardened off, they are ready to go into the garden.

Transplanting Seedlings Into the Garden

At this point the seedlings are still growing in media, not soil. If you grow in raised beds or containers that also use media instead of soil, just plant the seedlings where you want them.

If they are being moved into real soil there is a bit of a problem. Creating a hole in soil and filling it with the seedling in media creates a watering issue and makes it harder for roots to grow into your native soil. Solve these problems by following these steps.

Dig a hole a bit bigger than the seedling pot. Knock out the seedling and shake off most of the media right onto the ground. You don’t have to get every last bit, but try to get at least half off so that you expose some roots. Place the seedling in the hole, so that it is at the same height as in the pot. Fill the hole with your native soil.

Don’t amend the planting hole, and don’t fertilize right after plating. This will contradict a lot of what you read on the internet, but it is a much better way to plant.

Water the seedling right away or within an hour or so.

Tomato and pepper plants can be planted deeper since they will make roots along their stem.

If you are using Jiffy peat pellets, which I don’t recommend, remove the outer mesh – it does not decompose as promised by the manufacturer. If you are using peat pots, remove the peat pot before planting, since it does not decompose quickly enough to let the roots grow properly.

Treat newly planted seedlings just like any other new plant. Keep it well watered. If it is in the ground it probably doesn’t need to be fertilized. Containers will need some fertilizer. Fertilize vegetable gardens according to your soil test.

Perennials, Trees and Shrubs

The above transplanting instructions work well for all annuals and can also be used for perennials, trees and shrubs, what I will call permanent plants. However, I like to use a different method for this latter group.

Permanent plants tend to grow more slowly than annuals. At the end of the first summer some rock garden seedlings are less than an inch across. I find that when I plant small seedlings into the garden, I lose them. I prefer to keep them in pots at least the first year and maybe even the second.

Once these plants are hardened off, I continue to treat them like seedlings except that they get exposed to natural conditions; sun, wind, rain etc. I have an area for full sun plants and one for shade loving plants.

Once they are outside and growing well, I like to transplant them to get them out of the media. I want to start conditioning them to my soil, so I mix about half native soil and half media and use that to repot. For the media I just reuse the material in the seedling pots – it is still good as an organic source.

This does a couple of things for me. My soil has a higher clay content so I need to water less. It also holds nutrients better so I can reduce fertilizing. Thirdly, my seedlings get exposed to a different pH. Soilless mixes, especially peat based ones, are quite acidic and my soil is alkaline. By moving them into a mix of the two, they start getting used to a higher pH.

Since the seedlings are in pots, they will be watered more than if they are planted in the garden and will receive more fertilizer. You can fertilize them just like the seedlings, or add some slow release fertilizer to each pot. That way they only need to be fertilized once a year.

Most of my permanent plants will spend their first summer and winter in pots.

Overwintering Seedlings

Overwintering seedlings with pots sunk in the ground
Overwintering seedlings with pots sunk in the ground

One problem with keeping seedlings in pots is that they dry out quickly. To solve this, I like to bury the pots in the ground so that the soil comes up to the rim of the pot. Pots sunk in soil like this require much less watering.

I locate these pots near a water tap and a vegetable garden is a good place since you tend to go there regularly and can keep an eye on them.

Arranging the pots in rows with space between the rows makes weeding easier. Don’t let weeds grow because they will slow down the growth of the seedling.

I try to have all my pots in the ground like this before the hot mid-summer arrives. They stay in the ground until the following spring. You do not need to do anything special to overwinter them. They will stay nice and cozy, sunk in the ground.

Moving Permanent Plants Into the Garden

The seedlings are now a year old. Depending on type, they can still be quite small, or they might be quite large. The larger ones get moved into the garden, and the smaller ones continue growing in pots until I think they are big enough to plant out.

As they grow, they can be moved to larger pots so they don’t get root bound.

Once they are large enough, they are transplanted into a nursery bed where I can keep a clos eye on them, and water them more often. I can also evaluate their garden merit. They tend to stay in the nursery bed until they flower at which time I decide if they are worthy of being moved into the main garden. If not, I give them away or sell them.

The better plants are marked and transplanted the following spring. Here is zone 5, they could be moved in fall, but I find a spring move is more reliable.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

7 thoughts on “Seedling to Adulthood – Secrets for Growing Mature Plants From Seed”

  1. If I propagate roses at the end of July 1 of August what do I do with them in the winter. Do I leave them in the house over the winter and look after them or do I put them in the garden

    Reply
  2. Robert, sometimes I have trouble removing germinated seeds from the paper towel, they seem to be stuck to the paper. If the little seed head falls off can I still plant the root that is left behind on the paper. Is this a common problem.

    Reply
    • If the seed capsule fall off – then you can plant. If the stem is broken it won’t grow.

      You are leaving them too long. Plant sooner, as soon as you see roots. At that point it has not formed root hairs and does not stick very well to the tissue.

      Reply
      • Thank you for replying. Yes, I was probably waiting too long to remove them. Some seeds germinate very fast I’ve noticed.
        Take care.

        Reply

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