Correct potting is a very important part of begonia growing culture. The size and type of pot will determine to a large extent whether your plant will thrive.
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 that 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 mix regularly to keep the plant growing vigorously. The elements of your potting mix do break down over time and the mix will lose its draining qualities and airspaces to hold oxygen. You have certainly noticed when repotting plants the old mix, especially in the bottom of the pot, has turned to fine mud. The following is a list of tips and procedures for repotting.
When to Repot
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 roots hold all of the potting mix together then it’s ready to be moved up. If there is still loose mix 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 if done carefully.) If you have waited too long and the plant is really root bound you should gently loosen the roots before repotting.
Sometimes if you have a plant that doesn’t grow like it should, you may need to repot it. Even if it hasn’t filled the pot with roots, repotting back into the same pot with fresh mix may give it a burst.
Selecting the pot
Don't repot a begonia into a much larger pot. Generally, move up one pot size at a time. It’s better for the plant to be repotted more frequently in smaller jumps than to make one big jump.
If too large a pot is used, the soil mix stays too wet, then the soil sours, and the roots die.
For small plants only move them up in 1 inch increments until you get to about 6 inch size. After that 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. Also 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 and even then be careful with the watering.
What Potting Mix to Use
Mixes for outdoors
Many of you have your own potting mixes. 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 a mix that works perfectly for me. The reason it works well for me is that it’s very porous and allows water to drain well. It has enough organic matter to counteract my alkaline water as it breaks down, and my plants like it and grow strong healthy roots.
Some commercial mixes contain sawdust, which binds up the nitrogen in the potting mix so your fertilizer doesn't work properly. Some stay either too wet or too dry to work well for begonias. Even when mixed with other amendments such as perlite, oak leaf mold, etc., they just didn't seem to work as well for me as the mix I use now.
One important note is whatever mix you use it shouldn’t contain garden soil. You should only use mixes that are soil-less. This means they are composed of organic components but don’t actually contain soil. Garden soil contains soil born pests and diseases. Some growers also use peat or peat based mixes, which may work well where you have to water frequently because of extreme heat in the summer. They can be very hard to keep wet if they ever dry out completely if composed of mainly peat though. Nearly all growers use a potting mix that is at least partially peat based.
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 potting mix as you can. Then 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.
Mixes for indoors
For plants grown indoors you should always use a soil-less mix. Most of these are peat moss based. Most of the prepackaged mixes will do fine for begonias grown indoors. You may need to add perlite so some of them. You shouldn’t add things like oak leaf mold to mixes for indoors. Basically mixes for indoors are to hold moisture and food, and to give the roots something to hold on to.
How to Repot
Knowing how deep to plant.
When repotting, except for rhizomatous, rexes and some 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 year’s pruning. 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 potting mix 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 potting mix in the bottom of the new pot.
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 potting mix around the plant. The reasons 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 potting mix, and so the new mix will stay the same wetness as the old mix. My feeling is that if you only fill around the root ball loosely, when you water later, the water will flow too easily through the looser outside mix. So, it may not wet the root ball evenly. Some feel that if you pack the potting mix down you damage the roots too much…I haven't found that to be the case in the thousands of plants that I have repotted that way. More coarse mixes may need more packing down than light mixes like peat.
Fertilizing when repotting
When I repot my plants I always give them a fresh dose of Nutricote™. Nutricote™ is a time release fertilizer product. It is water release so gives a small amount of fertilizer every time I water. This makes up for any times that I miss my regular fertilizing regimen. There are many similar products such as Dynamo™ and Osmocote™. Different products may have different names or different availability in various parts of the country. Repotting is the easiest time to apply the Nutricote™ because you know they need it then so if you consistently use the Nutricote™ for every plant you repot then you won't miss any. Follow the directions on the Nutricote™ for amounts and how to apply.
Types of pots
I'm only going to cover the four major types of pots that are commonly used. They are clay, plastic and wooden pots, and wire baskets. Each has its merits.
Unglazed clay pots allow the roots to breathe because they are porous and allow air and water transfer. Glazed pots are similar to plastic in how they work. Many types of begonias grow well in clay pots, such as rhizomatous begonias which resent wet potting mix. The mix in clay pots dries out faster so clay will especially help growers who are chronic over-waterers. Many growers exclusively use clay pots and have found them to be the best pots to use. Nearly all begonias can be grown well in clay pots. The drawbacks to clay pots are that they are heavy and they cost more. But they look nice and last a long time. Trying to lift a 15 inch clay pot might be a challenge for most people. Once the plants get to a certain size sometimes clay pots aren't practical. Clay pots do come in every size and shape for nearly every purpose. Make sure to only use pots with drainage holes.
Plastic pots are the most commonly used pots. They are lightweight, relatively inexpensive, long lasting and work well for nearly all begonias. Watering and sizing correctly are the main problems with plastic pots. Unlike the other three pot types which are porous, plastic pots are not and hold water much longer. This usually isn't a problem and growers can usually find the correct size pot and potting mix to grow their plants in properly. Again, only use pots with drainage holes. Plastic pots have few bad qualities and they can also be found in any size and shape needed. White and light colored pots should be avoided because they deteriorate quickly in sunlight so aren't very long lasting. And they are frowned upon at begonia shows.
Wooden pots are still used but aren't as common. They work very well and most begonias can be grown easily in them. They aren't as easy to find, they don't have a good range of sizes, and they are expensive. They can be long lasting though and some can last for 10 years or more. Unless you have a specific design in mind you probably won't be using very many wooden pots. You should make sure to buy redwood pots if you want them to last. You can find pots of other woods that are only redwood stained and not actually made of redwood. They should have drainage holes but since they aren't water tight it isn't as essential as with the clay and plastic.
Moss baskets are still commonly used for their aesthetic value and also because begonias grow extremely well in them. They are expensive but do last a long time. Well, actually the baskets are cheap, it’s the sphagnum moss that can be expensive. They consist of a wire frame wrapped and lined with long fibered sphagnum moss. If you decide to make one use the green moss and not the brown floral type. After lining they are filled with mix and the begonia is planted as usual. They are usually hung up as hanging baskets. The moss allows for perfect air and water exchange and is nearly fool proof to grow in. They are a lot of trouble to make and wouldn't be practical for large collections but are nice for larger specimen baskets because they are lighter than any other pot would be of that size. Rhizomatous begonias can even make wonderful hanging baskets using this type of pot. They can be trained to grow completely around the pot to make a begonia ball. Also, even with the other types of begonias, they can be planted through the bottom of the pot also to make a ball. There are also various liners for the moss basket wire frames. Some are coconut fiber mats that are cut to fit the pot. These should be treated just like the moss covered baskets. There are other liners, which are plastic.
Wire baskets lined with plastic liners should be treated as plastic pots. You can also make your own wall pocket type moss pots for growing begonias on walls for semi-epiphytically. Attach chicken wire over a section of wood, line with sphagnum moss, and fill with a small amount of mix. Begonias, especially rhizomatous and trailing, will quickly cover the board. This method works well with all begonias considered epiphytic. I have a B. lymann-smithiithat has been growing on such a board for over eight years.
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