Color For The Shade – Growing and Caring For Tuberous Begonias

In warmer climates, the begonia is a perennial and can stay in the planting bed. Here in Saskatoon we must lift or dig them up and store them for the winter.

As I sit here during one of the coldest weeks on record writing the March 2021 blogs, I am happy to report that the first blog will be about starting and caring for tuberous begonias! Happy because tuberous begonias make me think about summer, warmth and beautiful, colourful double or semi-double flowers. For all the new gardeners, a tuber is a chunky potato-like storage unit for nutrients and water. The tuber provides the plant with energy for the current season and holds that energy during dormancy waiting for spring to help begin new growth. In warmer climates, the begonia is a perennial and can stay in the planting bed. Here in Saskatoon we must lift or dig them up and store them for the winter.

There are many types of begonias but the tuberous varieties are the queens of the garden. There are upright and trailing varieties; single , double, and ruffled blooms. The foliage is very attractive as well. And the best part of growing tuberous begonias? They love part to full shade! Morning sun is also acceptable. They bring a ton of color to the north and east areas of your garden and bloom non-stop until fall. I usually grow mine in containers or hanging baskets which are easy to move if the weather takes a turn for the worse.

Start your begonia tubers about 8 weeks before the last frost date which is usually sometime between May 15 and 31 in Saskatoon. Purchase tubers that are firm and free of obvious signs of rot. Large tubers produce large plants. Small tubers; small plants. Flower size is determined by the species , not the size of the tuber. Make sure you make use of the paper bags provided by separating each variety and color chosen and record the name and color on the outside of the bag. You can also throw a plant tag(s) into the bag with your tubers and use them to mark your pots. During the starting process make sure you label your pots as once you mix them up there is no way to tell them apart!

Four inch diameter plastic pots with drainage work well for starting your begonias. You can put the pots in a grower tray to catch the drainage water but do empty out the tray with each watering. A good quality potting mix with a few extra handfuls of perlite added to the bag will provide the moist but well drained environment they need to grow. Moisten the soil you will be using in a separate container and mix well. Fill each pot with this soil leaving a one inch space between the soil and the rim.

Now, which end of the tuber is up? The cupped side of the tubers may have sprouts, but if they don’t – the concave part goes up. Place one tuber on top of the potting soil in each 4 inch pot. Cover with about ½ to 1 inch of potting soil. Place your pots in the grower tray and put in a bright light location but not direct sun. Again keep soil moist by watering from the bottom allowing the plants to soak up the water. This will keep the water from accumulating in the cupped area of the tuber. I would also suggest to keep a small fan on low in the begonia room. Keep it pointed away from the pots. This will provide some air circulation preventing mold or rot as begonias are prone to this if kept too wet. Roots will form on all sides of the tuber and new growth will begin about a month after planting.

If the begonias get too leggy or spindly, cut the stems back to about 4 inches in height. 
This encourages bushier growth. I know it’s very difficult to do, but pinch off any flower buds as they form. A slight pinch of 20-20-20 in the watering can every time you water should help with any nutrient needs. Most good quality soils contain enough fertilizer to last for three months.
About two weeks before the last frost date harden your begonias off – put them outside during the day in a sheltered spot away from the wind and hot sun. Bring them in each night.

When the threat of frost is past gently transplant your plants into their final outdoor containers. Tuberous begonia stems are quite fragile and break easily. Again use a high quality, rich soil and containers with drainage. At this time you can mix a slow release fertilizer into the soil, then transplant. Place the plant so the point (s) of most of the leaves are pointed toward the outside rim of the pot. The blooms will then face outward as well.

This is also the time to stake your begonias with a 12-18 inch stake, pushing it into the soil carefully avoiding the tuber below. Fasten the stem to the stake with a soft garden tie or pieces of old nylons. Place in a semi-shade or shady location on the north or east side of your home in an area sheltered from the wind.

If you have not incorporated slow release fertilizer into the mix, you can fertilize once weekly with 20-20-20 at ¼ strength. Water at soil level, never over the top of the plant. Remove flowers when finished, cutting them off right at the stem. By the way, the boy flowers are the large double blooms and the girl flowers are the single petal blooms right below the boy flowers. To encourage more blooms, pinch the girl flowers off at the stem as soon as they form.

Preparing for Lifting and Storage- Gradually reduce frequency of watering and fertilizing by the end of August. Remove any flower buds and flowers. Wait for the foliage to yellow and then cut the stem back to about 4 inches. Remove the tubers from the soil and lay in a garage or shed to dry. When the stems break free from the tubers you can gently remove the excess soil and put the tubers in the sun for about four days. If the weather doesn’t cooperate you can dry them inside until the stems are loose. Do not wash the tubers. Check the tubers for any signs of rot or fungus and if all is good, place individually in a paper bag containing a bit of fungicide powder and shake to lightly coat. They then can be stored in very dry peat moss, sawdust or individually in their own small labelled paper bag. 

Place in a cool, dark location of your home. Check throughout the winter for any signs of rot or decay.

Some troubleshooting tips for tuberous begonias –

If buds and blossoms drop before their flower edges turn brown it may be a sign of too much water. You may be keeping your begonias too wet. Check drainage and reduce watering.
If the stems of a trailing begonia are growing upright and will not trail, the plants are not receiving enough light.