We have always been told that begonias like to have their root system fairly contained and it is disastrous to ‘overpot’ them. Most of the experienced growers who have written books or articles on begonia culture have stressed this point. I have often recommended using a smaller pot for growing a begonia in than one would see for an ordinary pot plant. Why? Because everyone says so and it has always been the rule. This is understandable in cool and or wet climates, or in glass houses which have a controlled atmosphere. Under such conditions the transpiration from the leaves and evaporation from the soil is greatly reduced.
The potting mix will hold its moisture for a longer period, and unless there is a large root system in relation to amount of soil, the water contained in the soil will become stagnant and the soil “sour”. I have found that it is important to pot newly rooted cuttings and small seedlings into very small pots because they usually have limited root systems and require oxygen (air pockets) to grow. It would be unwise to repot a begonia into a large pot at the beginning of winter when the growth rate is going to slow down and the plant will require very little moisture.
Excepting the previously mentioned conditions, I believe that in our hot, dry summers we can use larger pots than those we have been using. At the beginning of last spring my rexes in 12cm pots needed repotting. I know if they were put into 15cm pots that they would soon need larger ones and didn’t relish the thought of doing the job twice, so I rashly potted them into 20cm squat pots. I could hardly believe how rapidly they grew and they are still growing and looking lovely.
Over the years, I have always allowed seedlings and cuttings to grow quite large in 6cm pots before placing them in 12cm pots. Last season quite a few were potted on which I didn’t think were really large enough. The same results occurred – before I knew it they had to go into larger pots. Naturally if plants are going to grow larger and faster they will require more nourishment and I know that I do fertilise my plants more than the experts recommend. It has often puzzled me why they advise using liquid fertiliser at ½ to ¼ or even 1/8 strength. Even on tiny seedlings I use Phostrogen at full strength and they GROW.
Maybe compared to the USA we can use larger pots because of evaporation and stronger fertiliser because of more frequent watering and more rapid plant growth. Another contradiction I have discovered is in the depth of containers.
I had some pots delivered and it wasn’t until the carton was opened that I discovered that they were deep 12cm pots – not squat. They were used and the plants did well, soon sending their roots out through the bottom drainage holes. The roots of our plants may develop more quickly because they have to forage when our mix becomes drier between waterings. I was amazed to receive a letter from a grower who was lamenting the fact that she had pruned her canes right back at the beginning of LAST season and needed to have them looking good for a special event at the end of NEXT season. I always severely prune my canes in early spring and they are usually lovely by autumn. Sometimes they need to be trimmed in between. It must indicate that our plants do grow more rapidly.
Please do not start putting all your plants into huge containers. I would suggest that you experiment with a couple of plants that can be easily replaced.