Ground Prep
A little time spent in soil preparation will be well rewarded. If annuals have grown well in the bed before without fertilizer, then no additional fertilizer will be required for perennials beyond an annual maintenance light feed each year. If fertilizer is added to the bed, it’s best done well in advance of planting or at least mixed very well into the soil. It’s always safe to add a light application of fertilizer to the top of the soil around the plants several weeks after planting or when the plants have begun to grow.
All plant growing is a matter of judging plant response and deciding whether you are in the range of too much or too little (whether watering, spraying, fertilizing – or in regards to temperature, sunshine, whatever). What you’ll be trying to achieve is a border that’s established and can basically get along without any great effort each year.