Begonia

Light and exposure

Begonia likes light but dislikes scorching afternoon sun, which burns the petals and browns the foliage, especially on the large-flowered tuberous varieties. A partly shaded spot, under a tree with light foliage, on a terrace that only catches the morning sun or against a wall shaded in the afternoon, suits it beautifully.

The garden begonia, hardier by nature, copes with more sun under often hazy skies, provided the soil never dries out completely. Where the midday sun beats down hard, keep the open spots for well-watered bedding displays and save the shade for hanging baskets of tuberous begonias.

Watering

Begonia needs soil that stays consistently moist through the growing season, without ever becoming waterlogged. Water at the base and avoid wetting the flowers and foliage: standing moisture on the petals opens the door to grey mould, a real risk in a cool, rainy summer.

In a pot or hanging basket, the compost dries out quickly once temperatures rise and can need watering every day in high summer. The opposite problem, too much water around the tuber, causes it to rot within days, the single most common cause of death for tuberous begonias.

Soil and repotting

A humus-rich, light and above all well-drained potting mix is essential. Heavy clay soils, which hold water right through the cold season, need lightening with well-rotted compost and possibly some coarse sand before planting.

In a pot, always choose one with drainage holes and add a layer of clay pebbles or gravel at the bottom. A begonia left standing in waterlogged soil after rain will almost always develop tuber or crown rot.

Temperature and humidity

Begonia tolerates absolutely no frost, neither in the tuber nor in the foliage. Growth already slows below fifteen degrees and stops completely below ten.

Tubers are therefore only started outdoors, and plants only moved into open ground, once the last frost has passed and the soil has warmed, never before. At the end of the season the same rule works in reverse: at the first cold snap forecast, bring the plant in or lift it without delay, a single frosty night is enough to destroy both tuber and foliage.

Pruning and upkeep

Deadhead regularly: removing spent flowers is a simple habit that clearly extends flowering by stopping the plant from putting its energy into seed production. Pinch out the tips of young garden begonias too, to encourage a denser, bushier habit.

As the first frosts approach, cut the foliage back to a few centimetres above the ground before lifting the tubers. This trim makes drying and winter storage easier and lowers the risk of rot while the tubers are kept.

Pests and diseases

Powdery mildew, that floury white coating on the leaves, tends to appear in dry, still conditions when air does not circulate enough between plants. Grey mould, or botrytis, favours damp, cool weather, typical of oceanic summers, and attacks spent flowers left on the plant first.

Tuber rot, caused by standing water, remains the most serious problem because it is often irreversible. Slugs and snails happily go for the tender young shoots in spring, a ring of ash or a physical barrier keeps the damage down.

Propagation

Tuberous begonia divides easily at the tuber when growth restarts in spring: each piece needs at least one eye to sprout. The soft young shoots also root well from cuttings, either in water or straight into light, evenly moist compost.

Garden begonia is grown from tiny seed, sown on the surface under heated cover some three months before the planting-out date, since the seeds need light and steady warmth to germinate. It is a fiddly sowing job best left to patient gardeners, most people prefer to buy young plants or pre-started tubers instead.

Month-by-month care calendar

Green Hub tips

• Never plant or move begonia outside while a frost is still possible. A tuber or young plant caught by a late frost will not recover.
• Lift the tubers before the first autumn frosts, let them dry, then store them somewhere dry and frost-free, in peat or sand, at around ten degrees all winter.
• Choose light shade over scorching afternoon sun, especially for large-flowered tuberous begonias, which burn quickly in direct high-summer sun.
• Deadhead every week. It is the simplest habit for keeping a begonia in flower from early summer right through to the first frosts.