How to fix leggy seedlings

Fixing leggy seedlings most often comes down to correcting improper lighting, adjusting the temperature, pruning your seedlings, going easy on nitrogen heavy fertilizers and avoiding overcrowding your pots with too many seedlings.

Here today we will look closer at tall, leggy seedlings and in particular at how you can fix leggy seedlings.

What are leggy seedlings?

Leggy seedling are, simply put, seedlings or young plants that grow tall and spindly with skinny stems. Seedlings can have a rapid growth spurt for a number of different reasons where not enough light and too much heat are two of the more common factors.

It is easy to spot seedlings that have grown leggy as the stem is skinny and the whole plant appears stretched.

Are leggy seedlings bad?

A leggy plant is not a healthy plant. So yes, as you want healthy plants it is a problem. But it is not the end of the world as there are ways you can fix leggy seedlings.

So you do not want leggy seedlings as it is an indication that something is wrong. And leggy seedlings will not produce strong plants with sturdy stems and healthy crops but typically suffer from weak stems, irregular growth and poor yields.

What causes leggy seedlings?

Like with most things related to plants and herbs, prevention is always the best cure. Consequently, the best way to prevent leggy seedlings simply to understand what causes seedlings to grow leggy.

Ironically, you need to adopt a position where you treat your plants great but not too great. After all, nobody likes looking after spoiled kids, I mean plants.

And it is a tricky situation in that seedlings will actually thrive and grow leggy if the conditions are too perfect. I mean too perfect as in the temperature being a little bit too hot, but not hot enough to kill the seedling.

Or using a fertilized soil where the seed needs no extra nutrition to germinate. The seed may still germinate but the seedling will grow leggy as there is simply too much nutrition going around.

More examples of well-meaning mistakes include watering too much, refusal to prune or pinch the top of a seedling that has just started to grow or planting too many seeds in the same starter pot, just in case.

At the end of the day being too “nice” will increase the risk of raising spindly seedlings. Here, we will give you 7 actionable tips that will help you grow and develop healthy seedlings.

7 ways to fix and prevent leggy seedlings

Here are our top tips on how to prevent and fix leggy seedlings and plants.

All tips are tried and tested and will also help you develop a better understanding of why seedlings and plants grow leggy.

1) Provide more light

Insufficient light will cause young plants to stretch for enough light to produce the vital plant sugars that are needed for growth and survival. The process is known as photosynthesis.

And the light source is not important as long as the young seedlings get sufficient light.

Most gardeners start seeds indoors in early spring when there is not enough natural light. Instead you will have to rely on artificial light that often comes in the form of grow lights.

Grow lights to the rescue

Grow lights are effective but are still unable to provide as much light as the sun. You have to make up for this by having your grow light on for twice as long.

Yes, if your plant needs 6-8 hours of natural sunlight you need to run your grow light for 12-16 hours per day.

To give our seedling adequate light, we run our grow lights for 12 hours starting at 10 in the evening. This way any extra natural light during the day becomes a welcome bonus.

2) Adjust position of grow lights for more adequate light

If you notice that your seedlings or baby plants are leaning to one side you are most likely not providing evenly distributed light around the plants.

This is quite easy to spot with seedlings placed in for example a south facing window. Not all plants can get the spot with the most light and all the other plants will stretch and lean or reach towards the light.

The easiest fix is to simply rotate all the plants including the leggy plants to give all sides of the plants an equal exposure to the light.

An even better solution is to add extra light in the form of grow lights that give your plants an evenly distributed light.

3) Lower grow lights for more effect

Seeds, seedlings and plants need light to grow and thrive. During spring and summer the sun provides ample and evenly distributed light for free and, I guess, we are all forgiven when we take it “for granted”.

But it is not easy to replace sunlight with artificial light when growing from seed indoors. And here are two facts that I have learnt the hard way over the years:

  1. If you double the distance between the light source and the plant you will only get about 1/4 of the light as measured in Lux. You can test yourself by downloading one of the many free Lux meter readers that are available for your mobile phone.
  2. If your plants need 8 hours of natural sunlight you need to double the number of hours when using artificial lighting.

So what can you take from this? Place your light source close – but not too close – to fix leggy seedlings.

And how do you know if it is too close? Use your hand and make sure that the light does not emit too much heat for the plant.

4) Control the temperature

Heat mats can be a great way to help your seeds geminated. Simply place the pots with the planted seeds on the heat mat and wait for the first leaves to sprout.

But when seeds have germinated and you see first leaves it is time to turn the heating mats off or at least turn the temperature of the heating mat down.

Seedlings that grow in a high temperature environment will grow taller and spindly – especially if the light is not perfect.

If you do not have perfect light indoors it is better when seedlings grow in a room temperature environment (compared to a high temperature green house environment).

5) Overcrowded seedlings

When you sow seeds you need to remember that even tiny seeds grow into full size plants. Still it is so easy to plant too many seeds – especially with tiny seeds – when starting seeds indoors in pots and containers.

You fill your pots with potting soil or a seed starting mix and then sprinkle the seeds across the top. But it is hard to see the seeds on the soil surface. So you add some more seeds, just in case.

The result is of course pots with overcrowded seedlings fighting for light and nutrition. Now you end up having to thin out or transplant the seedlings into seedling trays too early and you risk loosing your plants.

Is there a better way?

Yes, absolutely!. Try using seed trays that are smaller in size and that by design are built to handle one (or two) seeds per pot.

6) Overfertilization causes leggy seedlings

Plant fertilizers are often balanced using a three number system called NPK.

Using fertilizers early, especially fertilizers with high levels of nitrogen (N), can lead to fast growth of greenery and development of spindly, leggy and tall stems.

If you want to use fertilizers to help fix leggy seedlings you should focus on fertilizers with a high last number, potassium (K). Potassium helps root growth and overall plant health.

The second number, phosphorus (P), helps promote the growth of flowers and fruit and comes into better use with more mature plants.

7) Top and prune your seedlings

Who in their right mind wants to pinch the top of a seedling that just started to grow a couple of weeks ago? You have waited for your seeds to geminate and then watched as the seedlings develop.

Still, as the seedlings develop you should be prepared to pinch the tops of any overly spindly stems growing tall.

Your plant will thank you for it and grow thicker and bushier.

Frequently asked questions

What are the main factors to consider with leggy seedlings

As you have already seen, seedlings can grow leggy for many different reasons. 

Still, the main factors are inadequate lighting in combination with a hot growing environment and the early use of fertilizers that are too nitrogen heavy.

Why should I thin tomato seedlings?

You need to thin your tomato seedlings to give the individual seedlings a chance to grow and develop into strong plants.

Overcrowded seedlings compete for light, nutrition, water and space when trying to develop into stronger plants with stems that can hold fruits.

If you do not thin out your seedlings you will end up with many weak, leggy seedlings and no harvest.

Why are my radish seedlings leggy?

Radish seedlings will grow leggy when planted too densely. You should plant seeds 5 cm (2 inches) apart and then be prepared to further thin out weak and spindly seedlings as needed.

If the weather is too hot it can also cause plants to grow tall and even leggy.

How to get thick stems on tomato plants

Firstly, healthy root systems promote the development of thicker stems. And transplanting tomato seedlings correctly will help the plant grow strong new roots all along the stem.

To transplant the seedlings, lift the seedlings gently out of the soil taking care not to hurt the root ball. Replant into soil in the bigger pot. Ensure that the all of the root and the stem up to the first set of leaves is planted into the soil.

You should also pinch the tops to promote stems growing thicker and stronger.

Lastly, using fertilizer with high level of potassium will help promote the growth of strong roots and overall plant health.

How to stop seedlings from stretching

Transplant your leggy and stretching tomato seedlings deeper into their soil, ensure that you are providing adequate lighting and that you are not using fertilizers that are too nitrogen heavy.

Why are my tomato plants so tall

You can get tall and leggy tomato seedlings for many different reason.

But first you need to remember that not all tomato plants are bushy. Some tomatoes plants, called indeterminate, will and should grow tall.

Tomato seedlings and plants can however also grow spindly and tall from growing in low-light conditions coupled with high temperatures.

Using high-nitrogen fertilizers can also promote growth of greenery and stems over flowers and fruit. 

Can you bury tomatillo stems

Much like tomatoes, tomatillos can grow roots all along the stem and seedlings should be planted deeply.

Meet the author: Mattias is an experienced gardener spending most of his free time on his knees among herbs, plants and garden vegetables. For the past two years he has been sharing gardening projects and how-to tutorials on the NordicLavender website and YouTube channel.