Growing Begonias Indoors
by Brad Thompson

       This article is to give you info on how to grow begonias indoors and in terrariums.  Except for the growers that are blessed with a climate where they can grow begonias outdoors year round, most growers have to learn how to grow begonias indoors. Sometimes it's only for the winter months, but for some plants and people it's a year around proposition.  Most houses, especially those with forced air heat, are not perfectly suited for growing begonias indoors. Most begonias can be grown in your home, however, if you are careful to provide each with its particular requirements. There are also a few tricks that will help make growing them in the house more successful.

1. Location and Light
  The first consideration is where exactly are the begonias going to live. The best light for growing indoors is to use fluorescent lighting. This will give you the most control over the amount of light your plants get. It doesn't have to be an expensive or lavish setup. Even a simple shop light with ordinary cool white tubes will suffice if you want to save money. A more expensive, but possibly more useful idea if you plan to start a larger collection of plants is to purchase a light stand. These can be bought from most mail order catalogs. The lights should run about 14 hours a day, but I have left lights on 24 hours a day and the plants did fine. The tubes should be just a couple inches above the tallest plant on the shelf.
Most of us also have natural light that we would like to take advantage of. There are begonias that will grow in nearly every window location except probably not well in a north window. Some plants in terrariums may grow fine in a north window, however. Different areas of the country vary as far as light intensity during the months of the year. You might have to experiment to find which plants grow in which windows. If you find that the plants are stretching, they need more light and if they are stunted and burning then they need to be farther from the window or moved to a window with less light. A good share of begonias will grow in a south window and most will grow in either an eastern or western exposure.

2. Pots and Potting Mixes
How your plants are potted and what soil mix you use will have a lot to do with how successful you are. Begonias hate to be over-potted or over-watered. It is hard to kill an under-potted begonia, but they can die pretty fast if over-potted. Don't move begonias to a larger pot until roots have filled the current pot. If you find that a certain plant never seems to dry out, it is probably in too large of a pot. Move it down to a size that fits the rootball after you've removed all the soggy wet mix.
For growing indoors you should always use a soil-less mix. Nearly all soil-less mixes are mostly composed of peat moss with additions of perlite and/or vermiculite. You can also make your own by mixing two thirds peat moss or a peat based mix with a third part perlite. A couple of commonly available peat based mixes are Sunshine and Fafard. If you use plain peat moss you should premoisten it before using because it can sometimes be hard to wet in the first place. When I use peat, I wet it with boiling water to get it slightly damp and then use it after it cools. I wouldn't recommend using any amendments you would commonly use outdoors such as leaf mold or manure, or even garden soil. These will cause your mix to stay too wet indoors and also invite diseases. A peat mix is the perfect mix for indoors since it drains well, but holds the perfect amount of moisture for the plant without staying too wet. I would use the same mix for begonias in terrariums.
As far as watering goes, a good pot for using indoors is the type called a self-watering pot. There are many brands and they work well for most begonias. You should also only use a peat-based mix for these types of pots; anything else will stay too wet. Allow the mix to dry out slightly before refilling the reservoir again.

3. Watering

   Regardless of what you have read in indoor gardening books, there are few plants that want to stay constantly moist. Besides, constantly moist is near impossible to achieve. Allow the surface of the mix to dry out slightly and then water thoroughly till water runs out the bottom when using conventional pots. Don't use saucers unless you fill them with pebbles so that the plant doesn't sit in water. You can use bottom watering, but empty the water out of the saucer after a couple of hours. Most begonias will also grow well using wick watering such as is used for African violets and gesneriads.

4. Humidity
Humidity is the biggest problem with growing begonias indoors. Although begonias don't like to be in wet soil, they do like humidity in the air. You can achieve enough humidity for many types of begonias by simply misting them daily. You can also use a humidifier. If you are growing plants under fluorescent lights a simple way to keep good humidity is to cover the plant stand with a plastic tent (make sure to use nonflammable plastic). They will still usually benefit from misting inside the tent because if the light fixtures are inside the tent they tend to keep it warmer and the air will dry out faster.

5. Terrariums
There are many begonias that are easier to maintain and that you will have greater success with if they are grown in terrariums. It's possible that your past attempts with terrariums have soured you on them, but if you follow a few simple rules you should do well with them. Most people keep their terrariums too wet and this is the most common reason for failure. You should only use a sterile medium for growing and this should only be damp.
The two most common mediums are the peat mix listed above or plain green sphagnum moss (don't use the brown florist type sphagnum). If you use peat moss, first put in a layer of perlite or pebbles in the bottom covered by a thin layer of mix. This will allow any extra water a place to drain. If you use sphagnum, wet it first with hot water and squeeze out the extra water before using. Peat and sphagnum are already basically sterile and if you take the added precaution to wet with boiling water before use, it will be even more so. There will always be some glass fogging after planting, but if it's excessive, crack the lid open until it has dried out enough that you can close tightly. Terrariums require less light than plants in pots and should never get direct sun. They should also not be fertilized often. Since the soil in a terrarium never gets flushed, the salts will build up and damage the plant.

6. Fertilizing
All plants that are actively growing do best with regular fertilizing. There are many brands of fertilizer formulated so you can fertilize every time you water by adding a few drops to the watering can. You can also just mix your own weak fertilizer such as quarter strength and use once a week.

7. Additional Notes
Begonias grown indoors can be pinched or pruned any time of the year. Any plants you bring indoors from outside will probably adjust easier if you trim them back. This will also give you cuttings to start a back up plant in case your plant doesn't do well with the change of conditions.
Indoor plants in nice warm conditions are very prone to mealy bugs. The easiest and least toxic mealy bug killer is plain rubbing alcohol. You can brush the mealy bugs with a cue tip or artist brush dipped in the alcohol and they die on contact. For a badly infested plant you can also put the alcohol in a spray bottle and spray the entire plant. It won't harm the plant. Avoid spraying the soil directly as a precaution.

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