How plants grow
Mighty oaks from little acorns grow – but how exactly? Here you can discover how plants stretch upwards as well as outwards throughout their lives and how we can help them grow up big and strong.
- Plants have growing zones, called meristems, in their stems and roots. Some create upward growth, making stems and roots longer, and some create outward growth, increasing girth
- Most plants grow from their tips, although grass-like plants grow from a meristem at their base
- We can use practices like pruning, pinching out and mowing to deliberately alter the growth of a plant to suit our garden
The process of growth
Growth occurs when cells divide, differentiate (become specialised to a particular function) and elongate. In plants, this occurs in growing zones called meristems and is controlled by three hormones: auxin, gibberellin and cytokinin.
The meristems in shoot and root tips are responsible for making a plant taller and longer, which allows leaves to reach sunlight and roots to spread out through the soil.
The meristems found inside stems and roots are responsible for making a plant wider and thicker, which ensures it can support the weight of its leaves, flowers and fruit.
How stems grow taller
In most plants, growth happens at the terminal (apical) bud at the tip of their stems. This bud produces auxins that suppress growth from any other (axillary) buds lower down on the stem, in a process called apical dominance.
Buds contain meristematic tissue and a concentration of hormones, which gives them growth potential if the part of the stem beyond them gets damaged. So, if the apical bud is removed, axillary buds are stimulated into growth.
An apical bud controls the growth of axillary buds lower on the stem
You can use this knowledge to produce bushier, more productive plants:
Did you know?
Palms don’t have buds along their stems, so if their growing tip is removed, they can’t produce replacement side-shoots and are likely to die.
Grasses and grass-like plants have their active growing zone (meristem) at their base rather than the tip. This means that when you mow your lawn, cutting off the leaf tips, the grass can continue to grow.
How stems grow thicker
As plants grow, their stems get thicker and sturdier to provide support. Soft green shoots gradually become woody and less flexible and tree trunks increase in girth.
This thickening is produced by a meristem, called cambium tissue, found inside stems. Its dividing cells produce more (secondary) xylem and phloem (tube-like transportation vessels) as stems grow and need more resources.
A cross-section through a stem
In green stems, cambium is spread through the tissue in bundles sandwiched between the xylem and phloem. In woody stems, over time, these join to become rings of tissue. This is sapwood, a soft outer layer lying between the inner heartwood and the bark.
Xylem vessels made in spring carry more water up from the roots than those made in summer and autumn, so spring sapwood is lighter in colour. A distinct line forms each year where the two tissues meet, and by counting the 'tree rings', it’s possible to estimate the age of a tree.
Annual growth rings at the base of a side branch
Trunk girth is also a useful indication of age. If you buy a young tree from a nursery, you’ll often find that the circumference of the trunk at 1m (3¼ft) in height is provided, to indicate the tree’s maturity and define its price.
The price of these rootballed trees will be determined by trunk girth
Bark, the protective outer layer on woody stems (and roots), is produced by another meristem, the cork cambium. Bark is several cells thick and is continually renewed by the cork cambium as the outer layer dries and cracks.
How roots grow
Roots grow into the soil searching for nutrients and water between soil particles. A root’s growing tip (apical meristem) and elongation zone sit behind a protective sheath, the root cap.
The root system of Festuca glauca
There are two types of roots:
Tap roots naturally grow straight downwards to seek out water and nutrients deep in the soil. They provide good anchorage and are often thickened with stored starch. In many trees, the tap root fades away over time as other roots develop.
As tap-rooted plants don’t transplant well, are hard to grow in containers and have fewer fine roots for water absorption, many tree nurseries undercut young trees, severing their tap root, to encourage more fibrous root growth. This makes them easier to manage and reduces transplant shock.
Fibrous roots form a fine, branching network with a large surface area for absorption. They are found just below the soil surface and collect water and nutrients percolating through the soil.
Around 90% of tree roots are found in the top 60cm (2ft) of soil, and as fibrous roots are easily damaged, it’s important not to compact this rootzone area, for example by parking cars under a tree’s canopy.
Plants with taproots, such as carrots, don't usually transplant well
Fibrous roots branch outwards into the upper layers of soil
Roots initiate branching internally, from an area around their central core where their vascular system (xylem and phloem) is, rather than from external buds like stems do. These branches grow at right angles to the parent root, to better explore the soil, and are exact copies of it, with their own ability to branch further.
As a plant grows, it keeps in balance the parts that make energy through sunlight (the green growth) and the parts that take up water and nutrients to feed it (the root network). Lifting a plant damages the fine feeder roots that absorb these key ingredients. That’s why cutting back top growth when you move plants can help restore the balance and ensure they settle into their new home well.
Your next steps
Now you know more about how plants grow, put this into practice to help your plants thrive:
- Pinch out the tips of summer bedding plants to improve their flowering display. Also see our guide to bedding plants
- Trim hedges regularly to increase their density. Most hedges benefit from trimming once or twice during the growing season
- Consider trunk diameter and look for a balanced ratio of roots to shoots when you next buy a tree – the thickness of the trunk gives you an idea of its age, which is useful when comparing prices, and a well-balanced tree should establish better. See our guide to buying trees
- Avoid moving plants with a taproot, as they have fewer fine feeder roots and often don't recover well if disturbed. This includes edibles like carrots, parsnips and dill, which are best sown in situ, and perennials like sea holly (Eryngium) and oriental poppies (Papaver orientale)
- Loosen the soil at the edges of a planting hole to make it easier for roots to grow and branch. See our step-by-step guide to planting a tree
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.