Regular repotting is very essential to the well-being of your begonias especially in their first few years
… as they are growing and maturing. Later, after the plants are mature and have already reached the maximum size you want them to be, you can let them go for a couple of years without repotting.
Yearly repotting will still be beneficial. Even with plants that you intend to keep in the same sized pot, you need to change the soil to keep the plant growing vigorously because the elements of your soil mix do break down over time. It loses its draining qualities and its airspaces needed to hold oxygen.
You have certainly noticed with plants you have repotted that the mix, especially in the bottom of the pot, has turned to fine mud. The following is a list of tips and procedures for repotting.
1. You should wait until a plant has filled its pot with roots before repotting.
If you gently remove the plant from its pot, you will be able to tell if it’s ready. If the plant holds all of the soil together then it is ready to be moved up. If there is still loose soil that stays in the pot after you pull the plant out, it needs more time. (pulling the plant out of the pot will not hurt it any.) If you have waited too long and the plant is root bound and the soil ball is totally filled with roots, then you should gently loosen it before repotting.
Note “Just a little reminder about potting down”: If you pull the plant out and it doesn’t have hardly any roots and most of the soil falls off, you may want to pot it into a new pot that fits the rootball to get it going again.
2. Selecting the pot: Don’t pot up into too large of a pot or make too big a jump in size. Generally, only move up one pot size at a time, because it’s better for the plant to be potted more frequently in smaller jumps than to make one big jump. An analogy would be, you could easily jump off a cliff if you did it in two foot jumps but if you did the whole hundred feet at once it would probably kill you (yes you’d be tired but you wouldn’t be dead after 50 two foot jumps.) If you try moving a 4 inch plant up to an 8 inch pot in one jump thinking the plant will grow bigger and faster, you would probably have the opposite effect, if the plant lived at all.
The problem with potting up too fast is not with the mix, it’s a problem with the amount of water that will stay in the mix. A 4 inch plant can’t use up the water in an 8 inch pot quickly enough. The soil stays too wet, sours, and starts killing what healthy roots the plant has. For small plants only move them up in 1 inch increments until you get to about 6 inch size, when you can make 2 inch jumps in size. Such as a 6 inch pot up to an 8, then an 8 up to a 10 etc.
An additional note; when moving up the smaller plants you need to remember that moving from a 3 inch round pot (for example) to a 4 inch square pot is a much larger jump than moving up to a 4 inch round pot. Don’t move up to the square that way unless your plant is very well rooted and has pretty good size. Even then be careful with the watering.
3. Your potting mix: Many of you have your own potting mixes, and if what you are using is giving you good results then keep using it. Mixes work differently depending on the growing conditions and your watering practices. I have experimented with various mixes over the years and have found that the only mix that works perfectly for me is the mix that most of our members use. The reason it works well for me is that; its very porous and allows water to drain well, it has enough organic matter in it to counteract my alkaline water as it breaks down, and my plants like it and grow strong healthy roots.
I have tried other mixes based on Supersoil, Unigrow, and Bandini and some other commercial mixes but none of them really worked well for me. Some commercial mixes contain sawdust, which binds up the nitrogen in the soil, so your fertilizer doesn’t work properly and some stay either too wet or too dry, to work well for begonias. Even when mixed with other amendments such as perlite, leafmold, etc. they just didn’t seem to work as well as the mix I use now.
Some growers also use peat or peat based mixes. They may work well if you live where you have to water frequently because of extreme heat in the summer. Peat mixes are very hard to keep wet if they ever dry out completely. If you ever buy a plant that is planted in peat or a mix that is very different from the one you use, remove as much of the original soil as you can and repot it in your own mix. If you leave the old mix, especially peat moss, it can dry out later in the middle of your pot and not rewet with normal watering. You won’t even know until the plant starts to suffer.
I’ve had plants that I left the peat soil on and planted them in baskets and they always seemed to be wilted looking even though they had just been watered.
When they were removed from their pots, I found that the couple of inches of fresh soil were fine but I had a large brick of dried peat moss in the middle that was as hard as a rock. When I watered, the water just ran around the dry middle and out the bottom, so the middle never got wet. You can see it’s important that whatever mix you use, the plant always stays in that same mix. If you change to another mix remove most of the old mix first.
4. When repotting, except for rhizomatous and tuberous, try to plant the begonia deeper than it was planted before. This is especially important if you have old stumps at the base of your plant from previous prunings. This will give your plant a fresh new look and also cause new bottom growth and new roots to form. If necessary, especially if you want to keep it in the same size pot, remove enough soil from the bottom of the plant so it can set low enough to cover up those stumps. Make sure to allow for at least a thin layer of new soil in the bottom of the new pot.
5. Filling with mix: There are differing opinions on this point but I’ll give you mine of course. I like to firmly pack the new soil around the plant. The reason I do this is; to remove large air pockets which will fill with water, to make sure the plant is in good contact with the new mix, and so the new mix will stay the same wetness as the old mix.
If you only fill around the soil ball loosely, when you water later, the water will flow too easily through the looser outside mix and may not wet the soil ball evenly. Some feel that if you pack the soil down you damage the roots too much but I haven’t found that to be the case in the hundreds of plants that I have repotted that way.
6. Fertilizing when repotting: When I repot my plants, I always give them a fresh dose of slow release fertilizer, such as Nutricote (TM) or Osmocote (TM). This makes up for any times that I miss my regular fertilizing (which I use in addition to Nutricote) and especially keeps all of my plants that I grow around the front of my house fertilized. I never make it out there with my liquid fertilizer, so that’s usually the only fertilizer they get all year.
Repotting is the easiest time to apply the slow release because you know they need it then. If you consistently use the slow release for every plant you repot (except for your rhizomatous begonias which may be burned by surface applied slow release fertilizer) then you won’t miss any. Follow the directions on the box for amounts and how to apply, with any fertilizer you use.
Hopefully this article will help remove any questions and doubts you have about repotting and will help you get a good start this year. I know there are probably some things I didn’t cover. E-mail me if you know of something I left out or other questions you might have.
Credits / references:
Brad Thompson | Brad’s Begonia World