They don't necessarily take as much water as you think,' she says. As you've probably guessed, there are exceptions. Some begonias, mostly smaller species, need the constant humidity of a terrarium. Some growers observe a rigid weekly or biweekly routine of fertilizing, ''and some of us just don't tend to get around to it as much,'' Leer says. Begonias are susceptible to mildew.
Some growers control mildew with fungicide.
Boardman uses a spray or drench of 2 teaspoons baking soda per gallon of water, applied as needed every four to five days. Don't crowd plants. Remove dead leaves and debris. Control mealybugs, a common houseplant pest, by dabbing or spraying with insecticidal soap or a mixture of rubbing alcohol and water. Many begonias are easy to propagate.