University of Maryland Extension: Garden Q&A
A: How about never? New woody plants have been fertilized in the nursery, and research shows that adding fertilizer at planting time slows root growth! (The most important thing you must do is keep them sufficiently watered for at least two years until they are established.)
More good reasons to skip fertilizer: pushing plants to grow quickly leads to weak growth susceptible to disease and drought. Weak wood breaks more easily in ice, snow or strong storms. Too lush growth is more apt to attract insects. Perhaps most surprising--deer will choose a fertilized plant over an unfertilized one, even normally deer-resistant plants.
Bottom line: mature established plants do not need to be fertilized with artificial fertilizers. Save your money.
On the other hand, composted materials are great. Also, when you mulch autumn leaves with your mower and leave them on your lawn or place them in your beds, they decompose and those nutrients feed your trees and shrubs. It’s the same as a forest revitalizing itself with its fallen leaves.
In addition, if you fertilize your lawn, some of the fertilizer reaches down to tree roots under the turf.
Q: My rhododendron has three stalks but it’s bare in the center and tall and leggy. If I cut it down to the ground, will it come back fuller?
A: Technically you can cut it almost to the ground—called a renovation pruning—and it will recover. However, it would be a tremendous shock to the plant and it may look bad for years.
Extreme weather conditions might knock it out. Instead, we’d recommend that you remove one trunk each year over a three year period, so that the remaining trunks can continue to carry on photosynthesis and keep the plant strong as it adjusts.