Fertilize established plant materials every few years. Apply fertilizer in the fall after the leaves have dropped, or early in the spring.
Never apply fertilizer in the late summer. This promotes new growth which makes the make the plant susceptible to winter damage by not hardening.
To prepare your plant material for winter you can apply phosphorus and potassium.
To prevent the fertilizer from drawing too much moisture away from plants, water both before and after fertilizing.