'Why are my seedlings leggy?' – the gardening experts explain, and provide practical solutions

Have your seeds sprouted weak and spindly shoots? This is what you need to do – and how to prevent it from happening again

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Q: I have started growing annuals indoors from seed, ready for planting out when the frosts finish. They're growing on my kitchen window sill, and germinated well, but now the seedlings are looking weak and leggy. What has caused this, and can they be saved?

A: One common reason behind leggy seedlings is insufficient light. On window sills, this is usually because the light is only coming from one direction. 'Plants grow towards light (a process that’s known as phototropism) and if this only comes from one side then leggy, stretched seedlings result,' explains Lucy Chamberlain, a gardening expert.

Leggy growth can also be encouraged by too high a temperature, explains John Negus, an expert from Homes and Gardens. When starting seeds indoors, they should be sown shallowly in proprietary seed compost at a temperature of around 64˚F, he says. 'If the temperature is higher, they grow so rapidly that they quickly become exhausted and collapse.'

John Negus
John Negus

John has been a garden journalist for over 50 years and regularly answers readers' questions in Amateur Gardening magazine. He has also written four books and has delivered many talks over the years on horticulture.

Headshot of Lucy Chamberlain
Lucy Chamberlain

Lucy was a Horticultural Advisor at RHS Wisley and has been Head Gardener on a 100-acre estate in England for many years, but writes regularly for titles such as The Garden, Gardeners’ World, The Guardian and Amateur Gardening. She’s also the author of RHS Step by Step Veg Patch, available from Amazon, which covers 50 types of fruit and veg. 

How to save leggy seedlings

In some cases, it's easiest to cut your losses and start again by sowing fresh seeds. However, if you want to try and rescue your leggy seedlings, you can try pinching them back. This can encourage bushier growth, depending on what type of plant it is.

Using clean scissors or your forefinger and thumb, shorten the shoots, leaving at least two pairs of leaves from the base. Then, place the trays or pots somewhere cooler in good light (you can follow the tips below for extra chance of success). Ensure you thin them out when needed, too.

When it's time to transplant them into larger pots, some plants, including tomatoes, can be planted slightly deeper into the soil, up to the first leaves.

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seedlings on windowsill

Plenty of light will encourage seedlings to grow strongly

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3 ways to prevent leggy seedlings

Whether you've tried to save your seedlings or have started a new batch, follow these tips to prevent the problem from happening again.

1. Move your seedlings to somewhere more suitable

Positioning is key for preventing leggy seedlings, so ensure you get this right for the best results. You want somewhere with plenty of bright, indirect light so that they can grow strong.

'Bay windows, porches, conservatories or greenhouses offer the best results,' says Lucy. However, ensure your greenhouse is clean: 'Dirty glass can block out anything from 30-50% of the sunlight hitting it.'

2. Turn the seedlings

If possible, give each pot a half-turn (180˚) every day, suggests Tim Rumball, a gardens writer. This will draw the seedlings in the opposite direction every 24 hours, he explains, ultimately resulting in upright growth.

leggy seedlings in tray

Ideally, your seedlings should get the same amount of light from all sides

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3. Make a reflector box

If you don't have a spot with perfect lighting conditions or the time to turn your seedlings daily, you can still avoid the seed sowing mistake of not providing enough light by making a simple reflector box. Tim Rumball explains how to do it:

  1. Cut a small, stout cardboard box in half diagonally (corner to corner) to make a tray with a base, a steep back and sloping sides. 
  2. Line the inside completely with aluminum baking foil.
  3. Set it on the sill with the steep back closest to you, then stand a tray of pots or a propagator on it. The foil reflects light coming through the window onto the back of the seedlings, resulting in straighter growth.

Alternatively, Lucy suggests simply placing a length of white card behind your window sill plants to reflect the light.

Tim Rumball gardening on an allotment
Tim Rumball

Tim Rumball was editor of Amateur Gardening magazine in the UK for 20 years. He is a keen grower of edibles and wrote a popular series called Grow, Cook and Eat. He is also known for his practical no-nonsense approach to gardening.

seedlings in tray

Strong seedlings will result in healthier plants, especially once they've been moved outdoors

(Image credit: Anne Swithinbank / Future)

With these tips, you'll be able to raise stronger, healthier seedlings that, with a bit of luck, will turn into robust plants. Just remember to harden off seedlings before planting them out, so that the drastic change in environment doesn't shock them.

Holly Crossley
Contributing Editor

The garden was always a big part of Holly's life growing up, as was the surrounding New Forest where she lived. Her appreciation for the great outdoors has only grown since then; over the years, she's been an allotment keeper, a professional gardener, and a botanical illustrator. Having worked for Gardeningetc.com for two years, Holly now regularly writes about plants and outdoor living for Homes & Gardens.