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
- How plants respond to light
- How plants respond to day length
- How plants respond to gravity
- How plants respond to touch
- How plants respond to cold
- Your next steps
How plants respond to light
Most plants grow and bend towards the light, especially sun-loving species. The ability of plants to grow towards a light source is called phototropism (from the Greek photo, meaning light, and tropos meaning turn
Light intensity affects the concentration of another hormone, gibberellin, which is responsible for controlling the length of stem between leaves (internode length):
Plants that are naturally adapted to growing in shade don’t have such a strong phototropic response. If you have a shady spot in the garden, such as under a tree canopy, select plants that will perform well there. In shady rooms indoors, opt for houseplants that originate from shady forests, such as aspidistras and ferns.
Houseplants adapted to low light levels will stay neat and compact
How plants respond to day length
Some plants are sensitive to different lengths of the day or night, a response known as photoperiodism. Plants use this chrysanthemums ability to help them adapt to seasonal changes in their environment and ensure key points in their lifecycle, such as flowering and leaf fall, take place at the right time.
- Short-day plants bloom when nights are getting longer and there are fewer daylight hours. Autumn-flowering 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
It can be difficult to distinguish day-length neutral plants as there’s often a complex interplay of several factors, including day length, temperature and/or light intensity, affecting flowering time.
How plants respond to gravity
The ability of plants to respond to gravity is called geotropism (from the Greek geo, meaning earth, and tropos meaning turn
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
The ability of plants to respond to touch is called thigmotropism (from the Greek thigma, meaning touch and tropos meaning turn).
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
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.
Use short stakes for newly-planted trees
How plants respond to cold
Cold temperatures can trigger flower bud development, especially in biennial plants such as honesty (Lunaria). Biennials spend their first year making a rosette of leaves, then flower once they have recieved enough chilly days. The period of cold needed varies from species to species, but most flower in late spring.
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.
Unseasonally low temperatures (and/or day length) can also cause premature flowering – bolting– which affects the quality of root crops and salads. Newly sown beetroot, for example, often bolts if temperatures fluctuate in spring. Once plants are triggered by low temperatures and turn their energy to flowering and setting seed, their roots stop swelling and this gives us a poor crop.
Your next steps
Now you know more about how plants feel and sense, put this into practice to help your plants thrive:
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
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.
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