Perennials are not fertilized nearly as often as annuals. Overfertilization can reduce flowering, Neier says. The goal is to use fertilizer before flowering to build a good strong plant and then maybe again after flowering to boost it back up. Fertilize lightly perhaps three times a year, he says. The fertilizer is then watered in.
Roses eat a lot.
If you're using a chemical fertilizer, start with a time-released type in the spring and reapply in midsummer, applying a complete rose food monthly in between times. Beyond the general guidelines, it's best to have your soil tested to see whether there are any deficiencies or overdoses in your garden. You may want to do separate tests in the lawn, the flower beds and the vegetable garden. Phosphorus, in particular, is often found in high levels in garden soils, Neier says. If a plant is not able to use it, the phosphorus can run off and become a groundwater pollutant.