Answer: Slow release fertilizers such as Osmocote are fine, as are general purpose granular fertilizers and organic fertilizers. Read and follow the label directions for how much to use. Top dressing with good quality compost is another option to use alone or in conjunction with fertilizer. And, using an organic mulch also helps feed the soil slowly as it breaks down over time.
The important thing to consider is whether or not you actually need to be fertilizing and if so which nutrients are specifically needed, such as nitrogen or phosphorus or potassium for example. Running a basic soil test is the only way to see what needs to be supplemented or if the pH needs to be adjusted. Your local county extension should be able to help you with the tests and interpreting the results based on what specific plants you are growing.
Your plants' appearance will also be a good indication of whether or not additional nutrients are needed. If in doubt, it is better to underfertilize than overfertilize. Fertilizing is not exactly a one size fits all situation. Some perennials grow well in a lean or low nutrient soil, others prefer a richer or more nutrient dense soil. Most will grow fine in average soil.
Most established trees and shrubs do not need fertilizing unless the soil is extremely poor. If you are fertilizing adjacent lawn area, that may be ample (or even too much) for the nearby trees and shrubs as well. Again, soil tests are the only way to determine the situation for sure.
I hope this helps.
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