Why are my seedlings so spindly?

This is a question which gets asked a lot. It happens when you're growing seeds in a propagator or on sunny windowsill. They come up, they look great, they grow - and keep on growing - until they are tall and wambly-looking. In the worst case scenario they fall over, go black at the base, or develop fluffy coats of grey mould.

Weak, over-grown seedlings are very vulnerable to fungus problems - the cause of the blackening or the grey fluff. There are products you can treat compost and seedlings with to prevent this problem (called "damping off"), but the best thing is to grow the seedlings strong and sturdy in the first place.

Seeds need two things to grow - as well as water and compost -  and these are heat and light. Heat stimulates growth. Light holds growth back. This is why plants bend towards the light - the shaded side grows faster than the well-lit side. This process is governed by a hormone produced in the plant's cells.

But tall, lush seedlings are weak. Their tissues are delicate and 'tender' - easily infected and easily damaged by handling. Short, sturdy seedlings are much healthier. Less heat and more light is the cure for spindliness. (You may see it in some of the more technical books referred to as "aetiolation". This is just a fancy word for spindliness.) If your seedlings are looking a bit leggy and lush, reduce the heat and give them more light. Some gardeners use reflective foil behind plants on a windowsill, to get them more light.

The process of getting plants used to less luxurious conditions is called "hardening off". This will be dealt with soon.