I live in Massachusetts and have some begonias that have been very happy all summer planted in a mostly shady area of my garden. The recent cold nights have caused the flowers to fall and the leaves to wilt, however they're still alive. I know begonias can be houseplants, but my question is whether or not I can successfully uproot them and expect them to survive the transition to a container and a warm house.
Yes, it's possible. Begonias are tender perennials, so if you bring them into suitable conditions for the winter, they can live for years in temperate zones. Here's what to do:
Prepare the area indoors. It must be well lit. If you don't have any large unobstructed (on both sides) windows to set them by (In winter, west or south facing is best due to the lower energy available from the sun), you will have to set up a grow light. LED lighting systems are expensive, but are the most efficient and long lasting.
Get the pots ready. The pot size will vary depending on the plant size, but you can try to get one that is about 2/3's the diameter of the plants' leaf spread. Get some high quality prefertilized potting mix, which will conveniently hold the plant over to the next season. Do not use garden soil.
Dig out the plants. Dig straight down amount the perimeter of the plant, in a circle about as big around as the pot. Pop the plant out of the ground. Carefully, remove the soil from the roots. Do not remove it all, and don't damage the roots. The goal is to have all the roots loose, but still coated in soil. Any lumps of original soil left in the pot will either become far drier or wetter than the surrounding mix, and can lead to disease and/or issues with soil acidity/nutrient uptake.
Put a layer of mix at the bottom of the pot, in a layer of about one inch. You don't yet want the roots to be in contact with the pot edge. Now, hold the plant with it's crown (where the roots meet the stems) about an inch below the top of the pot, with one hand. With the other, arrange the roots to radiate around the plant evenly, and move bunches of roots apart. Grab a handful of mix and sprinkle it into the pot, working it between the roots. Repeat until the soil is up to the plant's crown. While still holding the plant's crown at the right level, lightly tamp the mix, so that it will not settle later when watering. Add new mix as necessary.
Water the plant. Use distilled or spring water if you can, as tap water has properties which can eventually lead to health problems in potted plants. Water until all the mix is thoroughly moist, and water begins to run from the bottom of the pot. Then sit it in it's saucer.
Place the plant indoors. Do not set it by a heating vent, or a stove, or a frequently opened door to outside.
After this, some wilting is normal, but shouldn't last. Water the plant whenever the top 1/2" of mix dries out, and make sure to give it lots of light. If a heavy layer of dust accumulates on the leaves, you can clean them gently using a damp papaer towel, tissue, cotton ball, or cotton swab.