The Fine Art of Pruning
In order to have full, compact and healthy begonias, most need regular pruning. Regular usually means once a year. Pruning is also necessary to shape a begonia for a specific purpose such as to create a standard or a hanging basket. In this chapter we'll discuss all the different types of pruning and pruning specifics for the different types of begonias.
As a general rule, begonias grown outdoors in warm areas are pruned in the spring and those grown in cold areas are pruned in the fall. There are reasons for this. In warm climates such as Southern California, Florida, parts of Texas and other frost free areas begonias are grown outdoors all year round. In these areas begonias are usually pruned in the spring, March through June. The reasons for this are: Begonias pruned in the fall can winter-kill because of the wet winter conditions. Usually growers use the pruned parts for cuttings to start new plants and these don't start as well in the fall and are hard to carry over to the next Spring. Most canes start blooming later in the summer so if you prune in the fall you shorten the blooming season you would ordinarily have. In cold climates begonias are pruned in the fall for one simple reason: To make them smaller so they take up less house and greenhouse space when they're brought indoors. Most growers don't have so much extra indoor growing space that they can bring in all those huge full sized plants that have been growing outdoors all summer. Fall pruning brings these plants down to a managable size for winter. Cold climate growers also don't have the cuttings problem because all their plants are grown indoors so the plants from fall cuttings can winter over indoors too.
Before going on to the different types of pruning, there is one simple rule that applies to all pruning. Always prune directly above an outward facing bud. (see illustration A) For most plants this means pruning the plant so all the buds are facing away from the center of the plant. In the case of a trellised begonia or one against a wall, this means pruning so all the buds face towards the front of the plant.
General Pruning for Canes
In general practice, young plants are pruned hard and older plants can be either pruned hard or light depending on your purposes. Young plants (well established but only a year or two old) should be pruned hard. Hard means down to one or two nodes above the soil. (see illustration C) The reason for this is to force the plant to grow full. When you prune the plant hard it forces it to send up more new shoots. The first few years of pruning will determine how the plant will grow for the rest of its existence. If you let your plant grow tall and rangy the first couple of years, then it will always be tall and rangy unless you start it over again. With older plants that have already sent up a good number of canes, drastic pruning isn't as necessary. You can continue to prune hard every year if you want short compact begonias. If you want a nice tall cane, however, you will have to prune differently. Canes grow fast but it's hard to prune a cane down to a few inches from the pot and expect it to be a 5 foot tall specimen plant that season. With a mature cane you probably are going to want to do a light pruning just to keep it in shape. (see illustration D) The outer canes are pruned fairly low to encourage bottom growth so the plant won't be bare at the bottom. The inner canes are pruned at various heights, taller in the middle and lower with the outer canes. These instructions are for the typical upright cane. General pruning for creating hanging basket begonias will be covered separately.
General pruning for Shrubs and Semperflorens
Shrubs and semps are pruned slightly differently than canes. One reason is they generally grow much fuller. Shrubs also have an idiosyncracy in that most stems that are pruned will die back to the soil level. This mostly applies to the larger stemmed and leaved varieties and to nearly all hairy leaved varieties. A light pruning to shape will usually not cause this die back though. Many of the smaller leaved varieties have so many stems that pruning each stem individually can be very time consuming. You can prune these to a nice shape with hedge shears and then let them grow out. As a general rule, I prune Semps and the small leaved type Shrubs reasonably hard because they grow out so fast that a lightly pruned plant becomes overgrown before the end of the season. As they get too tall the stems fall over leaving the center open. Of course you can stake but it's much easier to just start out smaller in the spring. Another reason for pruning Semps and the small leaved shrubs back hard is because as a general rule they bloom all the way up the stems so that no matter where you prune the stems to, the new growth will still come from the bottom. A begonia stem can't send out a side branch from a node that bloomed so if it blooms all the way up the stem, after you cut it, it has no where to branch from except the lowest bud which is usually at the soil line. (see illustration E) With the larger leaved shrubs and hairy shrubs, a different pruning technique can be used. As I said before, with these types, stems that are cut usually die back to the pot. You have three choices. You can prune very lightly to shape to try and avoid the die back. You can just go ahead and prune hard since most will die back to there anyway. Or, you can wait for new growth before pruning. In the spring most shrubs send up a flush of new growth from the soil. At this time, once the new shoots are 6 inches tall or so, you can prune out all the old stems, leaving just the new shoots. This will be less of a shock for the plant but will have the same effect as a hard pruning. (see illustration F) You can use this method for the semps and smaller shrubs also. After you have removed all the old stems, leaving only the new, it will force even more shoots to come up, essentially giving you a completely new plant. This is the method I use for most shrubs. Another item to consider, especially with larger stemmed shrubs is that stems pruned above the pot branch out ugly. Another reason for cutting them back to the pot.
Like the large shrubs, many thick-stemmed stems die back to the soil if cut. If they don't, they leave an ugly stump where they branch. They never look nice after pruning unless the stems are cut completely down to the soil. As a general rule though, don't prune them too much. Very old, unsightly, or overly tall stems should be cut out completely but otherwise they should just be allowed to grow as they will. All of the thickstemmed have bare stems so you don't have to worry about forcing bottom growth, you can't. They will always have growth at the tips and be bare at the bottom. An exception is B.ulmifolia which you should treat as a shrub.
General pruning for Rhizomatous and Rexes
Yes, these types can be pruned also. The upright types can be treated somewhat like the thickstemmed but you can force bottom growth on them. Especially ones like B. crassicaulis, B. carolineifolia, and B. richii should be treated like thickstemmed. The other types can be trimmed back to stay inside the pot. Any rhizomes that are hanging over the edge of the pot can be trimmed back an inch or so inside the pot and you can also trim out any rhizomes that cross over each other. As with the other types this will force side growth that will make your plant fuller. It isn't as necessary to prune the rhizomatous as it is with the other types unless you are trying to restrict them to a certain size or they get out of shape.
General pruning for tuberous and semi-tuberous
Tuberous as a general rule don't need pruning. All your mistakes die back to the soil every year and you get to start fresh each spring. The caudex forming types (semi tuberous) can be pruned to keep them in shape if they get too large. Ugly growth can be trimmed down to the caudex.
General Pruning Tips and Rules
1. When pruning, especially canes and shrubs, you should cut out completely any stems that have multiple stumps from years of pruning. This will force up a fresh new stem to replace the ugly one. I call these older stems stair steppers. (see illustration G) Also prune out any other stems that are unsightly.
2. When pruning, always remove any stems or branches that cross over each other. They won't look better with time and it's best to remove them right away as you see them. Cut the offending stem back to a bud facing the other direction or if that isn't possible, cut it to the soil line and force it to start over again.
3. When pruning to make a begonia into a shapely hanging basket you need to force only side growth. Any new growth from the roots is almost always too stiff. I've heard many new and old growers refer to these strong stems coming up in their baskets as sucker growth. They thought the basket was trying to revert to something else. That isn't true. These strong shoots are the normal growth of the plant. (see illustration H) It's the hanging part that isn't its normal pattern of growth. Side shoots are always weaker than the upright shoots of a plant. To keep your basket as a shapely basket, when these stronger shoots come up from the base, prune them down right away to the lowest outward facing bud. This will force side shoots that will be weaker stems like the rest of your basket. The reason for cutting them down nearly to the soil is because you want them to branch down inside the plant. If you prune higher, they will branch higher. (see illustration I) This won't create a nice shaped basket. Remember a simple rule, if you want an upright plant, prune out side growth. If you want a basket, prune out upright growth.
4. As I said earlier, always prune to outward facing buds. If you just prune to heighth without regard to the buds, as they grow out, you will have branches growing every which way, crossing over each other. This will create an unsightly plant. Begonias on a trellis should be pruned slightly differently. You aren't concerned about branches crossing over because you want to cover the entire trellis. With a trellised plant, first tie as many branches as possible to the trellis. After you have done that, then you can prune the loose stems close to the trellis, down to an outward facing bud. When pruning a begonia that will stay against a wall, in a corner, etc. then prune all the stems so the buds are facing towards the front of the plant. This will force all the stems to grow against and away from the wall instead of into wall.
5. Although I have given you general times for pruning, you can really prune a begonia any time of year that you choose. The problem isn't with the pruning, it's with the watering afterwards. Since a begonia doesn't use hardly any water when all the stems are cut back and it has no leaves, it can be easy to overwater. This is one reason why fall pruning in warm areas can be a problem. The winter rains can overwater the begonia before it has a chance to grow back. In combination with colder winter temperatures, many begonias can winter kill easily. No matter what time of year you prune, you should always be careful with the watering until the begonia gets growing again. This is also the reason why many people have killed begonias when they pruned hard. It wasn't the pruning, it was the watering.
6. Safe Pruning. If you are a hesitant pruner, either because you are a new grower or just haven't ever pruned much, there is a safe way for you to learn. Instead of a light or hard pruning, do it in stages. This will get you used to doing the pruning until you learn just how much you can prune off without killing the plant. To prune in stages, first just take tip cuttings like you were taking cuttings to propagate with. This will force the plant to send out side growth and maybe even new shoots. After this happens prune a little more and farther down, maybe even a few stems at a time if you are really hesitant. Pruning just parts at a time you will eventually get the begonia cut down to where you want it. This will also give you a chance to see just what happens when you cut a stem, etc. Also, since you started with the tip cuttings, these will probably have already rooted and be potted up so that if you really make a mistake, you do have the cuttings to fall back on. You really shouldn't have those kinds of problems though, but better safe than sorry, especially when first learning.
7. If you have begonia standards (formed into small trees), they will require a little more thought when pruning. You want to prune them fairly hard because otherwise they'll get overgrown and not be able to support the long branches. On the other hand, make sure you leave quite a few buds so there will be something left to grow out after the pruning is done.
8. You can apply these pruning methods to many of the plants in your garden such as roses. I prune my roses and begonias the same way as a general rule.
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