Correct potting is very important to Begonia culture. The size and type of pot will determine to a large extent whether your plant will thrive.
Regular repotting is essential to the well-being of your begonias, especially in their first few years as they are growing and maturing. After the plants are mature and have reached the maximum desired size, they can go for a couple of years without being moved to a new pot.
Even with plants that you intend to keep in the same sized pot, the planting mix should be changed annually 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. 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’s roots hold all of the potting mix together, then it’s ready to be moved up. If a loose mix remains 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
Do not repot a Begonia into a much larger pot. Generally, move up one pot size at a time. It is better for the plant to be repotted more frequently in smaller jumps, than to make one big jump and over-pot.
Over-potting will lead to the soil mix staying too wet, souring, and rotting the roots. Begonias like their roots crowded and well-drained.
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. Do not move up to the square this way, unless your plant is very well rooted and a pretty good size; even then, be careful with the watering. Read more about Types of Growing Containers.
How to Repot
Know how deep to repot
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. Some like to firmly pack the new potting mix around the plant. This process ensures you remove large air pockets which will fill with water, 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. If you fill around the root ball too loosely, during watering 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. So, too hard a pack has it’s problems as well as does too loose a pack. More coarse mixes may need more packing down than light mixes like peat.
Read more on Soil Media
Fertilizing when Repotting
The addition of a slow-release fertilizer to the new medium will help build the strength of the plant and help it overcome the stress of repotting. These products are generally balanced, low formula, and control-released to give a small amount of fertilizer every time you water.
Extracted from: Original Article By Brad Thompson, Potting, Potting Mixes & Repotting