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Getting tuberous begonias off to a good start

If the bulbs are potted in March you should have plenty of blooms in June

One of my tuberous begonias last summer — covered with bloom.

We learn from our mistakes — sometimes! Last year I grew a couple of tuberous begonias after not having grown them for a number of years. I don’t like plants that demand a lot of care and tuberous begonias, although not terribly difficult to grow, do demand consistent attention and care. However, I was given a couple of them last spring so of course I planted them. The project was quite successful, but I learned a few things and will hopefully have even more success with them this year.

My biggest mistake last year was not getting the bulbs planted early enough. They were planted the beginning of April, so didn’t start blooming until early July. Although they bloomed non-stop after that and put on a great show, I wanted them to flower in June.

This year I am planting them two weeks earlier — about mid-March. I am also going to supply bottom heat under the planting tray for the first few weeks to encourage faster development of roots and top growth. The tubers seemed to take forever last year before I saw any sign of life. I am following the same procedure this year, planting the tubers about three cm apart in a shallow tray in a sterile soilless mix that is barely damp. I will “screw” the tubers in place by twisting them as I push them down into the planting medium. The tops will be barely covered with mix but the twisting motion will ensure good contact with the planting medium and that the tubers are securely in place.

Flowers of tuberous begonias are beautiful. The blooms can be single or double and can resemble roses or carnations (they are often labelled as “rose form” or “carnation form” by commercial growers). They come in deep rich colours ranging from purest white to deepest crimson, with orange, pink and yellow being some other common colours. There are even picote varieties, where the flower edges have a different colour from the main part; you might get a white flower with a pink edge.

There are upright varieties of tuberous begonias as well as pendulous types, so they can be grown in pots or hanging baskets as well as in the ground.

After the tubers are planted they should be placed in bright, indirect light (they can be started successfully under artificial lights in a light garden). The planting medium should be kept just barely moist and it will take a couple of weeks or so for roots and top growth to form. After there is top growth a weak 20-20-20 fertilizer can be applied. When top shoots are about 10 cm high the plants can be potted up individually; the size of the pot will depend on the type of tuberous begonia and how old the tubers are — older, larger ones will produce larger plants that require larger pots. Some of the cascading types also get quite large and demand large pots (usually hanging baskets). The pots should have adequate drainage holes and the planting medium should drain well, as begonias will rot if their soil is constantly soggy.

Don’t plant or move outdoors until all danger of frost is over, as they are not at all frost tolerant, so wait until outdoor temperatures are reliably warm. Tuberous begonias should be grown in the shade. Ideally, they will receive a bit of morning or late-day sun, which will promote more bloom, but they can be successfully grown in full shade. They will perform best in bright shade and will not do well in deep shade.

Begonias like to be sheltered from wind and yet they like good air circulation. One disease that often strikes them is mildew, and it more commonly develops in locations where the air circulation is poor. I grow my tuberous begonias on the steps of my north-facing deck which gets good air circulation but is sheltered from wind by nearby trees and shrubs. If white patches of mildew begin to appear on some of the leaves a fungicide should be used to prevent the disease from spreading. The plants are not tall, only growing to about 25 to 30 cm, so don’t require staking, although late in the season if they are heavy with bloom they may need a bit of support. Cutting off spent blooms keeps the plants attractive and prevents decaying flowers from landing on the leaves and promoting rot.

Some gardeners snip off the smaller female buds and just leave the larger male flowers, while others simply enjoy all that the plants produce. If there is a water feature in the landscape a begonia bloom can be cut and floated in water to create a nice feature. However you enjoy tuberous begonia blooms, they will keep coming all season right up until the plants have to be taken indoors in the fall. The tubers are easy to store. Allow the plants to dry down, remove the dead foliage and store tubers in dry peat moss in a cool, dark location.

Now is the time to get tuberous begonias out of storage and get them started into active growth. Don’t wait too long or like me last year, you will be wishing you had started them earlier than you did!

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