How plants sense and respond to the environment
Plants sense their surroundings and actively respond to the world around them. Here you can discover how factors such as light, temperature and gravity are responsible for making your seedlings bendy or your beetroot bolt and how you can work with these responses to help your plants thrive.
- Plants respond to external stimuli, like gravity, light and temperature, to give them the best chance of survival
- They react to these stimuli by producing chemical and hormonal signals that affect how they grow, adapt and reproduce
- Flowering is controlled by day length and temperature – gardeners can manipulate growing conditions to trigger or prevent flowering
How plants respond to light
Light intensity affects the concentration of another hormone, gibberellin, which is responsible for controlling the length of stem between leaves (internode length):
Houseplants adapted to low light levels will stay neat and compact
How plants respond to day length
- Short-day plants bloom when nights are getting longer and there are fewer daylight hours. Autumn-flowering chrysanthemums are a classic example. Although they’re known as short-day plants, it’s primarily the longer periods of darkness that trigger their flower buds to form
- Long-day plants naturally flower in summer – the hardy annual love-in-a-mist (Nigella) is an example
- Day-length neutral plants aren’t triggered into flowering by the length of day or night. Tomatoes are one example – if you sow them in early spring, the first flowers develop early too
Did you know?
Commercial growers produce cut-flower chrysanthemums all year round by using a little trickery. To encourage them to flower earlier, in summer, they use blackout fabric to artificially lengthen the night, mimicking autumn. Conversely, they shorten long nights with artificial lighting to encourage flower buds to form in winter.
How plants respond to gravity
Even the tiny, embryonic stem (plumule) and root (radicle) of a seedling can sense gravity, and will quickly orientate themselves after germination. This means it's not a deal breaker if you sow seeds or plant bulbs the wrong way up.
How plants respond to touch
Commonly seen examples of this response include:
- Tendrils grasping a support by curling around it – contact with the support stimulates a migration of auxin and causes cells on the outside edge of the tendril to grow faster than those touching the object
- Roots growing away from solid objects – they are negatively thigmotropic – so they choose the path of least resistance through the soil
Tendrils respond to touching a plant support
Roots grow around objects in the soil, like large stones
Plants may also respond to movement such as handling and brushing – known as thigmomorphogenesis. Plants that are physically disturbed produce more ethylene, which reduces stem elongation, resulting in stockier, more compact plants. Different species respond to varying degrees, but many young bedding plants, including pansies, are said to be especially responsive.
Outdoors, a strong breeze can stimulate this response. Plants grown in exposed locations stay short and sturdy so they are less at risk of wind damage. When you plant a young tree, be sure to give it a short stake so that it can flex in the wind and respond to its environment.
Did you know?
Tree stakes should ideally be attached at around one third of the height of the trunk. The stake's role is not to keep the trunk rigidly erect, but instead to stop the rootball moving in the ground so it can develop as an anchor. See our guide to staking new trees.
Use short stakes for newly-planted trees
How plants respond to cold
With many bulbs, flower buds develop inside the bulb in warm temperatures, but it’s only after a period of subsequent cold that the flower stem emerges from the bulb. Knowing this, we can ‘force’ spring bulbs, such as hyacinths, to flower earlier by chilling them for a specific period in September or October – see our guide to forcing bulbs for Christmas.
Biennial honesty needs a cold spell before it will flower
You can trick hyacinth bulbs into flowering at Christmas
Did you know?
In most parts of the UK, it’s best not to be over-eager and sow veg and salads if there’s a mild spell in late winter. Colder temperatures are still likely to follow and this could make your veg bolt. In general, it's much safer to sow in April when temperatures are likely to rise steadily.
These lettuces have bolted in response to cold temperatures
Your next steps
- Try to provide all-round light for seedlings on windowsills indoors – turn seed trays regularly or consider adding simple grow lights. See our guide to artificial lighting
- Give climbing plants with tendrils the support of canes and wires. Tie them on to start with, giving them contact, and they’ll respond by wrapping their tendrils around to hold on themselves
- Use short stakes on newly planted trees. See our video guide to planting and staking trees
- Avoid sowing most veg when cold weather is still likely – crops such as beetroot and Swiss chard may bolt
- Force spring bulbs for an early indoor display. Put pots of bulbs in a cool dark place, at around 12°C (54°F), for six to ten weeks (depending on the type of bulb), before bringing them into a warmer location to flower
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.