With the exception of a few ravenous bushes such as everblooming roses, most mature trees and shrubs in good soil don’t require fertilizer. That is proved by those in forests growing with literal wild abandon. Of course, they have the advantage of being fed by their own decaying leaves, a natural process that homeowners thwart by raking.
But assiduous homeowners also fertilize their lawns, so trees and shrubs near that turf are likely to receive all the nitrogen they need without additional feeding. However, the bark mulches used so widely these days can sometimes leach nitrogen out of the soil beneath them. So you may want to mulch with compost instead, which actually adds nutrients.
If your trees and shrubs appear pale or stunted, have your soil tested to determine both its pH and its nutrient level. You can purchase a mail-in kit for that purpose from an agricultural extension office, and the results will inform you of what changes you need to make. The problem actually may be related to excess acidity or alkalinity, in which case you may need to apply lime or sulfur rather than fertilizer.
While adult trees and shrubs usually don’t need fertilization, you may want to feed younger ones to encourage them to mature more quickly. It’s best not to start immediately after they have been planted, though, as you want them to concentrate on putting down roots then. So, although it’s tempting to press food on them like an overindulgent grandmother, give them a year to make themselves at home first.
Even then, be careful how much you give them. Not only can excessive fertilizer contribute to water pollution via run-off, it also may promote an overabundance of soft new growth which is more vulnerable to pests and disease. That gut—er, glut—might, in fact, be compared to the soft waistlines on humans also caused by overfeeding. Because you definitely don’t want such “fat” to develop at a time when it shortly will be harmed by winter freezes, refrain from fertilizing your trees or shrubs in late summer or early autumn.
Since run-off from your turf can also affect your landscaping, you probably should avoid fertilizing your lawn during that time period too. The cool season grasses most commonly grown in the north generally need only a couple feedings per year anyway. So it makes sense to apply those at the same times often recommended for fertilizing trees and shrubs—mid-spring and/or late autumn.
You gardeners who prefer organic methods may want to leave your grass clipping on your lawn after you mow or apply a thin layer of raked-in compost to that lawn once or twice a year, as such supplements can reduce the amount of fertilizer you require. You generally will need about twice the amount of an organic fertilizer—such as Espoma Organic Lawn Food or Dr. Earth Super Natural Lawn Fertilizer—as you will of an inorganic one such as Lebanon Pro or Scotts Turf Builder, due to the lower N-P-K ratio of the former. However, because organic fertilizers release their nutrients more slowly, they are less likely to burn your grass.
As mentioned earlier, roses and other shrubs that bloom all summer should receive frequent meals to keep them up to that task. The makers of rose foods often recommend applications every other month during the growing season, but be sure to cease those after the beginning of August.
Always read fertilizer instructions carefully and—if they give you a choice of amounts—opt for the lower one first. You always can add more later, if necessary, but taking it back is much more problematic!