I have a really huge planter that I have put blackberries in. I have had it now for three years and only blooms on one branch. It is in direct sunlight. I turn the pot and water till it comes out the bottom. I cut the plant down to the roots and still only get berries on one branch. I use an organic fertilizer and sometimes put in coffee grinds. What am I doing wrong?
The container size should be at least 5 gallons or larger. The wider the better.
You should provide a trellis for support.
They will need more frequent watering, check the soil daily and water when the first inch of soil is dry.
Nutrients are washed away with all the watering so you need to provide a complete balanced fertilizer regularly.
A slow release fertilizer can be used in early spring, or use regular fertilizer once a month.
They will need regular pruning and training.
Divide the plants every couple of years, when they are dormant--in the winter.
This is a good time to replace the soil also.
If you see a white crust start to form on the top of the soil, give the pot a good rinse to help rinse all the built up salts.
If you have been fertilizing heavy in order to try to encourage fruit, the soil may have to much nitrogen.
You can try adding some bone meal to add phosphorus.