You can help your plants thrive by finding a consistent source, preferably free or at a very low cost. Once you see the difference fertilizing has on your plants you'll never look back. It serves as a little boost in the beginning of spring once it's warmed up a bit. You may or may not already notice fresh new growth emerging from your plants but then it would be the perfect time to add some fertilizer.
Fertilizers come from many sources and in different forms such as Powder, Granules, Compost or Liquid, etc. The main or let's say 'Macro' nutrients to look for are N for nitrogen, P for phosphorous and K for potassium. Think Up, Down & all around. N= Up/green growth, P= Down as in roots/establishment of the plant/flowering and K= All around/particularly aiding in flowering. We can get into micro nutrients later. They are far more numerous and detailed.
Make sure when you look at ingredients they sound like everyday items (Bat guano, Chicken/Cow/Steer manure, kelp, rice hulls). The synthetic type of fertilizer is not recommended because of the detrimental effect the runoff water with elevated fertilizer (Specifically Nitrogen) levels has on everything. A synthetic fertilizer bottle will read more like, 'Derived from: Ammoniacal nitrogen, Urea, Potassium Phosphate, Feric oxide', etc.
If you can avoid runoff water with fertilizer these 'synthetic' types can be used for ornamental & flowering plants.