Tomato seedlings not growing | 7 Top Causes and Cures

Tomato seedlings will not grow if they run out of nitrogen, are not getting enough sunlight or warmth. Seedlings bought from in small pots should planted out into your garden bed or a larger pot straight away as the soil will not support them to grow bigger. Improve garden soil first with aged cow manure and compost to feed tomato seedlings.

Top 7 causes and cures of tomato seedlings not growing

1. Not enough light and warmth

Tomato seedlings need light and warmth to grow well and cold conditions in spring can slow their growth. Use a seed growing kit or place them in a greenhouse to provide them with the warmth they need.

Tomato seedlings can stop growing if the weather is too cool and slow their growth. Surround tomato seedlings planted out in your garden bed with mulch. This will help to insulate their roots and protect the soil.

Tomato seedlings are ready to plant out after they have grown their second set of leaves.

As the weather warms, the plants can suddenly spring to life, so don’t give up. Give them a liquid nitrogen fertilizer like fish emulsion and this can encourage new leaf growth

2. Not enough water

Tomato seedlings that are allowed to dry out can quickly stop growing. Small tomato seedlings will need regular water to stop them drying out. Any dry spells can stop their growth and it can be tricky to get them back on track.

3. Tomato seedlings are ready to be planted out

Tomato seedlings will stop growing in their small pot as they run out of nutrients from the seed raising mix. They will need to be planted out when they grow their second set of leaves as they will need to take up more nutrients from the soil.

I use sugar cane mulch around my tomato seedlings.

The seed raising mix that most tomato seedlings are grown in is low in nutrients and the seedling can quickly run out of nitrogen. This can cause the tomato seedling leaves to turn yellow. This is a key sign that they should be planted out into a garden bed or pot with good quality potting soil.

Before planting tomato seedlings into your garden bed, improve the soil with aged cow manure, compost and worm castings if you have them. This will provide longer lasting nutrients for your seedlings.  

Tomato seedlings will quickly run out of nutrients from seed raising mix so plant them out or add a liquid nitrogen fertilizer

4. Frosts

A sudden frost can stop your tomato seedlings from growing. If they are planted out in your garden too early in the season or if there is a sudden late frost in Spring, the seedlings can be damaged or may die. If the tomato seedlings survive the frost their leaves can be damaged enough that they will stop growing.

Choosing when to plant seedlings out in the garden can be tricky as it is hard to know when the last frost will be. When the weather warms in Spring, your tomatoes should be ready for a life outdoors, but you may have to watch the weather and cover them the night before with shade-cloth if frosts are predicted.

A greenhouse can protect young tomato seedlings from frosts.

5. Root bound in pots

Tomato seedlings that become root bound in pots will stop growing. Most tomato seedings will need to be planted out into a larger container once they have grown their second set of leaves. When you remove the seedling from the pot, check the roots and tease them out gently at the bottom if they have become tangled.

6. Transplant shock

Moving tomato seedlings from one pot to another can stop their growth for a short period of time. Damaged roots or just moving the plant from one area to another can slow their growth.

A great solution for transplant shock in tomato plants is to use seaweed solution. Seaweed root tonic are great for helping roots to repair and improving the soil over time. It can help to grow mor good bacteria in the soil which can make nutrients available for the plant.

Worm juice is another great root tonic for tomato seedlings. Dilute worm juice in water the color of weak tea and water it onto your plant. This will help transplant shock and can get your tomato seedling growing quicker.

7. Potting soil has run out of nutrients

Tomato seedlings that are planted out into potting soil will need an extra boost of nutrients after 3 months. Most potting soils will only have enough slow release nutrients added to support plants for three months after planting.

Add pelleted chicken manure to your seedlings around 4 weeks after planting out to give them an extra boost of a range of nutrients. Tomato plants grown in pots will need to have more nutrients added at least every 2 months to keep them growing strong.

Tomatoes grown in ground soil can reach their roots deeper and find nutrients in the lower levels of the soil while tomatoes in pots are completely dependent on you.

Make sure that tomatoes are grown in a container that is 12-18 inches deep depending on the tomato type. For more on this check out my previous article on how deep to make a planter box for vegetables.

Check out my other article: Pot size for tomatoes to find out details for each type.

Tomato seedlings not growing – FAQ

How do you fix stunted tomato seedlings?

Tomato seedlings that have been stunted can be fixed by adding a liquid nitrogen fertilizer every 2 weeks to encourage good leaf and stem growth. Plant stunted tomato seedlings out into your garden bed to give them more sun, nutrients and add mulch.

How can I make my tomato seedlings grow faster?

To make tomato seedlings grow faster apply liquid nitrogen fertilizer every 2 weeks for the first 8 weeks. Then once the plant has grown 2 feet high, stop adding nitrogen and add a fruiting fertilizer to increase the potassium level. This will encourage flower and fruit growth for lots of tomatoes.

What is the best fertilizer for tomato seedlings?

The best fertilizer for tomato seedlings after they have grown their second set of leaves is pelleted chicken manure. This is slow release, rich in nitrogen and a range of other nutrients and will feed the seedling as it grows its next set of leaves. Pelleted chicken manure will speed tomato seedling growth and help it to establish leaves and stems ready to grow fruit.

Why are my seedlings not getting any bigger?

Seedlings will not get any bigger if they are not getting enough light, water, nutrients or are root bound. Remove them from their seedling pot or tray, tease out the roots and plant them into a garden bed or pot with good quality potting soil.

Should tomato seedlings be in direct sunlight?

Tomato seedlings once they have grown their second leaves can be moved into direct sunlight. Make sure the weather is not above 80 degrees Fahrenheit and it is not too windy. Move seedlings that have been grown in shade gradually into bright light, for a few hours in the morning and move them back into part shade in the afternoon.

Hardening off seedlings is the gradual process of moving them into sunlight and getting them used to the full sun. This process can be done gradually to avoid any leaf burn.

Tomato seedlings not growing – Summary

Tomato seedlings will stop growing when they run out of nutrients so if you are buying them from a nursery, plant them out into your garden or a large pot straight away. This is very important and you can slow tomato seedling growth if they are left for too long in their small pot.

Sunlight, warmth, nitrogen and water are the keys to good tomato seedling growth.

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