I've had my wax begonias for at least a few years. They both began to grow beautifully at first but began to die as time went by. (each at its own pace)
I'm not sure what's wrong, they don't seem to die off completely as new leaves and stems continue to emerge but the old ones also die rapidly.
The bigger one still seemed rather healthy, but some leaves are turning transparent and dying off. Then slowly the stem dies. I'm not sure if it's supposed to be like this.
The small one looks to be sick with entire areas dead, but I don't know what the problem is. New stems and leaves are emerging but they also die a few days/weeks after emerging.

Does anyone know what could be wrong with them?

Difficult to say what the problem is. My first thought was excessive heat - here any double flowered semps I have died a month ago from the heat! But seeing your location I think it may be a combination of soil too dense - what I see in one edge of the pot is very little perlite - and too much water. The plant also looks as though the light may be too low. Semps really like very high light, many even taking considerable sun, particularly in a cool climate.

It is hard to diagnose without a photo that shows the entire plant. including its pot. I suspect it is a watering problem. What is your watering routine?

When plants are stressed, they usually shed their older lower leaves first while they continue to push out new growth with the expectation that conditions will improve so the plant can survive.

LAbegoniac said:Difficult to say what the problem is. My first thought was excessive heat - here any double flowered semps I have died a month ago from the heat! But seeing your location I think it may be a combination of soil too dense - what I see in one edge of the pot is very little perlite - and too much water. The plant also looks as though the light may be too low. Semps really like very high light, many even taking considerable sun, particularly in a cool climate.

Thank you for your reply!
I don't think exessive heat is a problem due to the cool temperatures in my area. However in the winter it's usually really cold and I usually leave them beside the window. I'm not sure if it's too cold for them but they already survived several winters...
It's possible that the soil may be too densed, would it help if I gently dig a shovel into the pot to loosen the soil? Or should I do something else?
As for execessive water, I doubt that it's the issue as it's so dry here that a few days without watering cause the soil to clump up and leave a crack between itself and the pot. So I try to keep the soil moist.
I used to expose them to high light but I thought they needed indirect sunlight so I changed the location. I've just moved them into bright sunlight, we'll see if that helps.
Thanks again!

WillC said:It is hard to diagnose without a photo that shows the entire plant. including its pot. I suspect it is a watering problem. What is your watering routine?

When plants are stressed, they usually shed their older lower leaves first while they continue to push out new growth with the expectation that conditions will improve so the plant can survive.

Thank you for the reply!
Here are some more pictures, I'm not sure if they are useful:






The first is a picture of the smaller wax begonia, the second one is the larger.

I don't actually water by a schedule. I have a lot of plants so I check on them every other day to see if the soil is dry. Sometimes I water every few days, sometimes a little longer apart.
Does irregular watering affect these plants a lot?

One of the possibilities is an infection by Vine Weevil larvae, I have had that with wax Begonia's, the symptoms you're describing would fit that..
I would inspect the root balls of the plants.
This is how the white grubs and the adult beetle look like..:

Are these plants indoors in pots year round? I grow wax begonias outdoors here (also Canada) and they are fine in full sun, partial shade or fairly heavy shade at this latitude. They'll be covered in flowers, as they are now, until the first frost kills them.

Prune back all of the leggy stems. If the roots are healthy, you should get healthy new growth starting where you make the pruning cuts and growing upward from there.