Scarborough, planted in large containers, in shade/partial shade, with good potting soil.
1. Approx. length of time to reach maturity of 12’x15′ ?
2. Will this plant perform in our hot Ontario summers ?
Most info available about this plant from southern USA States.
Rieger begonias grow quickly. However, you don’t mention the size of the plants you transplanted into the large containers. If you start with small plants, it will take a few weeks longer for the plants to reach their full width of 30-38 cm (12-15 inches) and height of 20-30 cm (8-12 inches). You’ll likely have nice bushy plants within 6 weeks, although I did not locate any literature that specifies time from planting to achieving maximum size. The plants should be spaced 30 cm (12 inches) apart, to give them room to grow. The begonias also need good air circulation, so should not be crowded in by other plants.
Also, the type of care the plant receives will determine how they perform. For example, the plants need adequate water and moist, well-draining soil, and fertilizer. If the soil is too dry, the blooms may wilt and might not recover. A slow release fertilizer used according to package directions is straightforward, but use caution as as too much fertilizer can damage the plants. Remove the blooms as they fade, to encourage additional blooms and discourage pests. Try and avoid watering the leaves, as this could encourage fungal growth.
It is important that the plants get lots of indirect sunlight – not direct sunlight. Siting them in part shade should be ideal. If you find they are not growing as well as you’d like, move them to a spot where they receive more indirect sunlight — the full shade you mention may be too much for them.
As for whether the plants will do well during our hot Ontario summers, this may be a concern. These begonias don’t like temperatures that are too high or too low. They do well with temperatures between 15-21 degrees Celsius (60-70 degrees Fahrenheit) during the day, and 10-21 degrees Celsius (50-60 degrees Fahrenheit) at night. With temperatures above 27 degrees Celsius (80 degrees Fahrenheit), you may see flowers and buds dropping.
Note that Rieger begonias are often used as houseplants, and are beloved in part because they flower during winter — in order to bloom, they need cold temperatures and short days. They are kept as outdoor plants year-round in USDA zones 9-11, which explains in part why most of the information you found on-line relates to the southern US.
Good luck with your begonias – I have a feeling that they will be lovely!
May 30 2022