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GREENFILE COLUMN: Tuber Time

Dahlias
Dahlias - Mark Cullen

By Mark and Ben Cullen

The gardening season is here.

Take tubers for example, more precisely, dahlia and begonia tubers. If you over-wintered them in your basement or your vegetable crisper, now is the time to take them out and start them for a summer full of colour.

If you didn’t hold yours over from last fall, now is the time to buy them from your favourite garden retailer. We urge you to do this soon, while the selection is at its best.

If you start tubers now you will end up with the longest possible season of colour. The later you start the shorter the flowering period.


Here is how to get started:

Dahlias. The large, cactus flowering dahlias are remarkable, mostly for their pie-plate size and their suitability for cutting to bring indoors. The smaller pom-poms are just as popular but are used more as border plants. There are many varieties of dahlias in between, in a riot of colours.

Remove the mother-tuber from its winter storage and separate the long, finger-like tubers from the main stem. Each one will grow up to 10 times their original size in good soil and sunshine. A tuber about 10 to 15 cm long and 3 to 6 cm thick will produce a good-sized plant come late spring. Pot each tuber up in to a gallon size pot now. Use a quality potting mix that drains well like Pro Mix.

Push the soil down around each tuber and firm it into place about 5 cm deep. No need to worry about ‘which side is up’, place the thick root in horizontally, you won’t go wrong.

Water it well and place in a sunny window. Gallon sized pots are a bit large for most windowsills. We put ours on the floor at the sliding door at the back of the house. As the plants push through the soil, in about four to six weeks, turn them every few days to encourage even growth.

Tuberous Begonias

Tuberous begonias are famous for their large double-sized flowers that look much like roses. Because they grow so well in part shade (not full shade) they are a great substitute for roses on the north side of your house or under the dappled shade of trees. The hanging varieties are very showy in hanging baskets.

Placing your begonia tuber in the soil right-side-up is important. The convex side (the bulged portion) goes down while the concave or indented side faces up. Spread a quality peat-based seed starting mix in a seed starting tray soil about 5 cm deep. Leave room at the rim of the tray for water to percolate down into the root zone of the soil. Push each tuber into the soil and give it a twist. Be firm.

Then apply a one or two cm layer of soil over the top of the tubers. Water well and let the soil become dry to the touch between watering.

Place the tray on top of your refrigerator, where the warm heat rising up the back of it will help to encourage early root development. In about four to six weeks, gently tease each tuber out of the soil and plant them, new roots, green shoots and all, into 10 cm pots.

Place the potted begonias in bright sunlight until planting time in late May or early June.

Note that both dahlias and begonias are ‘hot crops’ that enjoy heat and wither in cold weather and cold soil. Plant them out when the soil has warmed up to about 15C.

Mark Cullen is an expert gardener, author, broadcaster, tree advocate and holds the Order of Canada. His son Ben is a fourth-generation urban gardener and graduate of University of Guelph and Dalhousie University in Halifax. Follow them at markcullen.com, @markcullengardening, on Facebook and bi-weekly on Global TV’s “National Morning Show.”

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