Begonia Care: Light, Water and Feeding for Bedding, Tuberous and Foliage Types

Which of the three you are growing decides everything

The name covers plants with almost nothing in common in the garden. The fibrous bedding sorts are the small waxy leaved plants used for mass planting: tough, self cleaning, happy in more sun than the others, and grown fresh from young plants or seed each year. The tuberous sorts grow from a flat brown tuber, carry the big camellia shaped flowers people buy them for, and die down completely at the end of the season. The foliage sorts are grown indoors for their leaves, spread by a thick creeping rhizome at the surface, and flower thinly on wiry stalks that most growers cut off.

Telling them apart takes one look at the base of the plant. A dense fibrous mat of thin roots with no swollen storage organ is the bedding type. A flat disc, dished on top, sitting at or just under the compost surface is a tuber. A knuckly horizontal stem creeping across the surface with leaves rising from it is a rhizome, and that plant is a foliage begonia. There is also an upright group with jointed bamboo like stems and hanging clusters of small flowers, which behaves like a permanent houseplant and is treated as a foliage type for watering, but is cut back like the bedding types.

Almost every begonia question that gets a contradictory answer online is a question asked without saying which group the plant belongs to. Watering, feeding, whether to lift it in autumn and whether it can take midday sun all differ by group, and none of them differ much within a group.

Light: bright, but very little direct midday sun

Tuberous and foliage begonias burn. The damage shows as pale bleached patches on the leaves that later dry to thin brown paper, always on the leaves facing the light source, and it happens in a single hot afternoon rather than gradually. Two to three hours of early morning sun with bright shade for the rest of the day is what these two groups want. Under the edge of a tree canopy, on an east facing wall, or a few feet back from a south window behind a net curtain are all right.

The fibrous bedding types are the exception and are far more forgiving. The bronze leaved ones in particular take full sun all day provided they are never allowed to dry out completely, and they flower more heavily in that position than in shade. Green leaved bedding sorts sit between the two and do best with sun until midday.

Too little light is easier to miss than too much. A begonia short of light does not look ill; it stretches, with long bare stem between the leaves, leaves that turn to face one way, and flowering that quietly stops. On a patterned foliage begonia the first symptom is the pattern fading to plain green, because the plant is building more chlorophyll to compensate. Move it brighter and the new leaves come back patterned, though the faded ones never recolour.

Turn indoor and shaded begonias a quarter turn every time you water. They lean towards the light faster than most houseplants, and a plant that has grown lopsided over a season cannot be straightened, only cut back.

Water the compost, never the crown

Begonia roots are fine and shallow and sit in the top few inches of compost. They rot quickly in standing water and they die back quickly in a dry pot, so the target is compost that is damp through and never saturated. Water thoroughly until it runs from the base, then wait until the top inch (2.5 cm) is dry to the touch before watering again. In a warm room or a summer basket that is every two or three days; in a cool autumn it can stretch to a week or more.

Water at the compost surface and keep it off the leaves and out of the crown. Begonia leaves are thick and hold droplets for hours, and water sitting where the leaf stalks meet the stem is the most common start of the soft brown rot that takes out a whole crown overnight. Tuberous types are worst affected, because the dished top of the tuber holds water like a saucer. Where a tuber sits at the surface, water around the edge of the pot rather than into the middle.

An overwatered begonia and an underwatered one look similar from the top, both limp with yellowing lower leaves. Push a finger two inches (5 cm) into the compost to tell them apart. Wet and limp means the roots have gone, and the plant needs to be taken out, the rotten roots trimmed off and the rest repotted into fresh barely damp compost. Dry and limp recovers within hours of a proper soak.

Feed lightly and often rather than heavily

Begonias are moderate feeders and they respond badly to being overfed. Too much nitrogen gives large soft brittle leaves, stems that snap at a touch, and very few flowers, and that soft growth is exactly what powdery mildew colonises first. A balanced liquid feed at half the strength on the bottle, given every ten to fourteen days through the growing season, suits all four groups.

Switch the flowering types to a feed higher in potassium, on the pattern of a tomato feed, from the point when the first buds are visible. That is the single change that makes the most difference to the size and number of flowers on a tuberous begonia, and it also firms up the growth enough to hold the heavy blooms without the stems folding.

Foliage begonias want less again. Feed them at quarter strength once a month while they are putting out new leaves, and stop entirely from late autumn until the days lengthen. Feeding a foliage begonia through a dark winter produces a couple of long weak leaves on stretched stalks and nothing else, and those stalks then flop over the pot rim for the rest of the year.

1. Start feeding once the plant is growing, not before A newly potted plant or a freshly started tuber has enough in the compost for four to six weeks. Feeding into unrooted compost pushes salts around roots that cannot yet take them up.
2. Halve the recommended strength Use half strength twice as often rather than full strength on the interval given. Begonia leaf edges brown and crisp from fertilizer salts long before the plant shows any sign of being hungry.
3. Move to a high potassium feed at first bud From the first visible flower bud, swap to a tomato style feed. Flowers come bigger, stems come stiffer, and the growth is less attractive to mildew.
4. Flush the pot every six weeks Water heavily enough for a good volume to run out of the base, twice over. That washes out accumulated salts, which show as a white crust on the compost and on the pot rim.

Humidity yes, still air no

All begonias prefer humid air, and the foliage types visibly suffer without it, browning around the leaf edges in a dry heated room. The obvious answer, misting the leaves, is the wrong one for this genus: it wets the foliage, and wet begonia foliage in still air is how both powdery mildew and gray mold get started. Raise the humidity around the plant without wetting it, by standing the pot on a tray of damp gravel with the pot base clear of the water, or by grouping several plants together.

Air movement matters as much as the humidity itself. A begonia in a humid corner where nothing moves will mildew; the same plant in the same humidity with a gentle draft across it stays clean. Outdoors that means not planting them shoulder to shoulder against a fence. Indoors it means not pushing them into a closed alcove, and cracking a window on mild days. In a greenhouse it means leaving vents open overnight even when it feels counterintuitive.

Cold and wet together is the combination that kills. Below about 55F (13C) with damp compost, the fine roots stop working, the plant stops drinking, and rot follows within a week or two. As autumn nights cool, cut the watering back before the temperature bottoms out rather than after.

Write down the date you first see any powdery white bloom on the leaves, and where the plant was standing. That date and that position repeat closely year on year, and knowing them lets you thin the planting and open the airflow a fortnight in advance instead of reacting to it.