Bolting: why plants run to flower early
A central stem shooting up with a flower cluster at the top. Once it starts you cannot reverse it — but you can usually work out why it happened.
What you see
A stem extends rapidly from the centre of the plant, carrying small leaves and a cluster of flower buds at the top. Leaf production stops. In leafy crops the leaves turn bitter and leathery; in root crops the root goes woody.
It happens fast — often within three or four days of the first sign. Lettuce, spinach, coriander, pak choi and radish are the crops most likely to do it.
Why it happens
Water temperature above the crop's limit
The single most common cause indoors. Lettuce bolts above roughly 24 °C in the root zone regardless of variety, light or nutrients. Spinach and coriander are similar. If your reservoir is warm, bolting is not a nutrient problem and no additive will stop it.
Day length
Many crops measure the hours of light and flower once a threshold is crossed. Spinach and radish respond strongly to long days. Running lights for 16 hours in summer mimics midsummer and triggers flowering in long-day crops.
A check to growth
Drying out, a sudden EC spike, root damage or transplant shock can all push a plant into flowering as a survival response. A plant that stops growing for a few days and then bolts was usually stressed, not warm.
Nitrogen running out
A plant that becomes nitrogen-deficient late in its cycle will flower early. This is more common in a small reservoir that has not been changed.
What to do
You cannot reverse it. Cutting off the flower stem buys a few days at most, and the leaves have already turned. Pull the plant and replant.
Then fix the cause before the next cycle:
- Cool the reservoir. 18–20 °C for lettuce and spinach. This is what the chiller is for.
- Shorten the photoperiod. 12–14 hours for leafy crops rather than 16.
- Keep the reservoir stable. Change it on schedule and top up with plain water rather than letting EC drift up.
- Choose bolt-resistant varieties. The catalogue lists slow-bolt types for lettuce, coriander and spinach.
Bolting at day 22 — was it the reservoir temperature?
What temperature are people running lettuce at in summer? Mine goes over at 24 °C regardless of variety.
All problems
Six things go wrong in a small reservoir. The index covers each.