In Massachusetts, putting begonias outside for the summer can have real benefits. Of course there are a number of factors to consider, beginning with light. Sun lovers, like canes, appreciate direct sun, which helps to trigger flowering. Shade lovers, like rhizomatous, can be more of a problem but I have found that such begonias not only tolerate, but can benefit from one or two hours of direct sun. This will dry out the pots a little faster and this must be taken into account. Water as usual, though the higher temperatures will necessitate more frequent watering. Rainy periods are more of a problem. Begonias typically need to dry out between watering, and rainy periods may not allow that. In such cases it may be necessary to either shield the plants from the rain or bring them inside during extended rains, as wet soil risks poor growth, or worse, root rot and the loss of the plant. One concern, which never materialized for me, is the threat of insects. I found that mealy bugs are less of an issue outside. It’s possible that other insects eat them, but whatever the reason, they were clearly easier to deal with. Other insects like fungus gnats simply fly away and never reappear. Of course the risk of insects infecting the plants is real and it is necessary to be prepared to spray the begonias if necessary. The permethrin/insecticidal soap mixture on the Buxton website is a good choice. Although less noxious than other effective pesticides, the recommended spray is still a pesticide and the cautions recommended here and by the manufacturers should be followed.
One problem not experienced indoors is animal damage. Deer and rabbits can devour a Begonia and chipmunks love to dig in the pots. When deer or rabbits eat a Begonia it is not just that a few leaves are missing, but especially with deer the plant is likely to be torn apart. Steps or a stand will inhibit rabbits and possibly even the chipmunks. but they are no obstacle to deer. If the neighborhood has deer it is best to cage the plants somehow. I have chanced a few begonias on my front steps and some years it may be no problem, but in others it turns into a disaster. Caging prevents that.
With the issues of herbivores and excessive rain, there is another alternative and that is some kind of portable outdoor greenhouse. These are available in different sizes and types. The one shown here is a fairly large walk-in unit purchased on sale from Ocean State Job Lot for approximately $90. With the polyethylene cover removed, it disassembles into pieces so the whole unit can be stored compactly in my garage for the winter. It takes 2 hours to put together in the spring and take down in fall. The white polyethylene enclosure allows it to be put in full sun for 3 to 4 hours without damage to my rhizomatous begonias. The light levels are much higher than those under my indoor lights where they are grown otherwise, but the very bright light only lasts for a few hours, while in my basement the lights are on 15 hours a day. The leaves are not burned and there are no dry edges on the leaves. The plants are on shelves and with the enclosure zipped up for the night, deer and rabbits have to find their meals elsewhere. Chipmunks cannot climb the shelves so they are no problem. Similarly heavy rains are no problem. To protect against high winds during thunderstorms, I put bags of sand under the bottom of the frame to weigh it down.
If you put your begonias out for the summer, then they must come in for the fall. I bring the begonias in when night temperatures start to dip into the 50s. I have never experienced a reaction to begonias being put out for the summer, but the same cannot be said for bringing begonias in for the fall. Conditions indoors are quite different from outdoors and the plants react by sharply reduced growth and even some leaf loss. They recover over the next few weeks, though sometimes it can take as much as a month. That said, I have always come out well ahead by putting the begonias outside for the summer. They grow much more and are more bushy than if they are kept indoors all summer.
I open it during the day if it is especially hot, because it does get warm in there. It’s in the turnaround in my driveway and gets between 3 to 4 hours of direct sun before the trees shade it, so it’s then in open shade. It’s great. The white polyethylene cover diffuses the sunlight greatly, which is why it works so well for the rhizomatous begonias. That said there are two shrub-like begonias in there and they are doing well also.