Begonia Growing Calendar: Starting Tubers, Summer Care and Storing Over Winter

The shape of the year, by type

The tuberous begonia calendar is the one with real deadlines in it. The tuber is started into growth indoors in late winter, potted up and hardened off through spring, planted out only once frost is past, flowers all summer, and is then lifted before the first frost and stored dry until the following late winter. Miss the lifting and the tuber turns to mush in the first hard freeze; start it too late and the plant flowers for half the season it could have.

The fibrous bedding types run a simpler annual calendar: sow or buy young plants in late winter, grow on cool and bright, harden off, plant out after the frosts, shear once at midsummer, and clear away at the first frost. They can be overwintered as cuttings on a windowsill if you want to keep a particular plant, but most growers start again.

Foliage begonias have no dormancy at all and no calendar in the same sense. Their year is a growing season from spring to autumn and a resting period through winter, when watering is cut back, feeding stops entirely, and the plant simply holds. Repotting and dividing are spring jobs for them, done as the light returns.

Late winter: waking the tubers up

Start stored tubers into growth ten to twelve weeks before your last expected frost. Set them in a shallow tray of barely damp compost or coir, hollow side upward, spaced so they do not touch, and press them in only far enough to hold them steady. The dished side is the top and the rounded side goes down; a tuber planted the wrong way up will still grow but wastes weeks doing it, and the shoots come up bent.

Hold the tray at around 65F (18C) somewhere with good light. Water very sparingly indeed at this stage, just enough to stop the compost drying out entirely, because a dormant tuber sitting in wet compost rots rather than sprouts. Pink shoot buds appear on the upper surface within two to three weeks, which is also the moment you find out which tubers survived the winter; anything still bare and hard after four weeks, or soft anywhere, should be squeezed and binned if it gives.

Once the shoots are two to three inches (7.5 cm) tall, pot each tuber individually into a pot an inch or two (3 to 5 cm) wider than the tuber, with the top of the tuber level with the compost surface, and begin watering properly. If a tuber has produced four or five strong shoots, rub off the weakest two with a thumb; three strong stems give better flowers than five thin ones.

Spring and summer: out, up and flowering

Nothing goes outside until the last frost has passed and nights hold above 50F (10C). Harden off over ten to fourteen days, starting with two hours in a sheltered shaded spot and building to a full day, always bringing the plants back under cover overnight. Begonia foliage grown soft under glass scorches in one sunny afternoon if it is put straight out.

Plant into bright shade with two or three hours of morning sun at most, spacing upright types eight to twelve inches (20 to 30 cm) apart so that air moves between them. Push a slim cane in beside each upright tuberous plant at planting time; going back later means driving a cane through a root system and through a tuber. Water in well and keep the compost evenly damp for the first fortnight while the roots move out.

Through summer the routine is short and repetitive: feed every ten to fourteen days with a high potassium feed once buds show, remove the two small female flowers from every truss as they appear, snap out spent flower stalks at the base weekly, and keep water off the foliage. Powdery mildew usually arrives in the second half of summer, so this is when to thin any crowded growth rather than waiting to see the white patches.

Record the date you started the tubers and the date the first flower opened. Those two dates together tell you exactly how many weeks your own conditions need, and that is what lets you start the tubers a fortnight earlier next year if you want flowers by midsummer.

Autumn: lifting and storing the tubers dry

This is the part of the year that decides whether you have plants next season. As days shorten the plant stops flowering and the leaves begin to yellow. Cut the water right back and stop feeding completely, but leave the top growth attached. Those yellowing leaves are pulling stored food down into the tuber, and every extra week they stay on makes a bigger tuber and a stronger plant next year.

Lift before the first frost, not after. A frosted tuber is often already ruined even when it looks intact. Dig or tip out the whole plant with soil still clinging to it, and lay it in a tray somewhere frost free, dry and airy with the top growth still on. After two to three weeks the stems will have withered enough to pull away from the tuber with a light tug. Do not force them; a stem torn off leaves a wound in the crown of the tuber that rots in store.

Once the stems are off, clean the remaining soil and old roots away, and let the bare tubers dry on the surface for a few more days. Then pack them in a box of dry sand, vermiculite or coir, not touching each other, and keep them somewhere dark, dry and frost free at 45 to 50F (7 to 10C). A garage or a spare room is right; a heated living room dries them to a shell, and a cold shed freezes them.

Check the box once a month through the winter. Firm tubers are fine. Any that has gone soft, or shows blue green mold, comes out at once, because rot runs from one tuber to its neighbors in a closed box. A tuber that has shriveled slightly but is still hard is only dehydrated and will start normally in late winter.

Winter: the plants that keep going

Foliage and cane stemmed begonias do not go dormant and need managing rather than storing. Move them to the brightest position you have, because winter light indoors is a fraction of summer light even by a window. Cut watering back to whenever the compost is dry halfway down, stop feeding entirely, and keep the room above 60F (16C) if you can. Cold plus wet compost is what kills indoor begonias in winter, and it kills them in a fortnight.

Do not repot at this stage, however pot bound the plant looks. Wait until the days lengthen and new growth starts, then repot and divide. A begonia repotted in midwinter sits in a volume of wet compost its reduced root system cannot use.

Late winter is when the cane stemmed types are cut back. Take out one or two of the oldest tallest canes at two or three joints above the compost, before the new growth starts. New canes come from the base in spring and the plant stays furnished low down instead of turning into bare poles.