Our rule of thumb is to fertilize with a pellet form, slow released fertilizer i.e. 12-6-8, in spring, summer and fall.
The early spring fertilization will give the plants a good jump start to the beginning of growing season.
The mid-summer feeding will give them another boost to continue growth and to ward off any insects or disease. The fall fertilization will aid in strengthening the plants through the winter months.
We do not recommend fertilizing in the winter because fertilization promotes growth, making the tender new growth vulnerable to frost damage.