Last week Jeff wrote about the dangers of using “balanced” fertilizers, especially in reference to phosphorus content. Comments quickly followed about using fertilizers in many situations – on farms, in container plants, on trees – and so on. One of the latest comments came from Nick and began “I don’t usually recommend fertilizer for perennials or woody plants to consumers. In most cases they aren’t needed.” And this leads into today’s topic.
Many of the horticultural practices we use in our gardens and landscapes have, unfortunately, been derived from agricultural crop production. Whether you’re growing a field of wheat, garden tomatoes, or containerized shrubs your goal is maximizing crop production. By its nature, this is an unsustainable practice because it requires continual inputs of water and nutrients at higher levels than would naturally occur.
But this is not how you should care for landscape trees and shrubs, and why Nick’s comment was a good one. You don’t need to routinely add fertilizer to these plants; they don’t need it to grow normally. What we should be doing in landscapes is preventing nutrient deficiencies. Once you have a soil test in hand, you’ll know what nutrients may be too low (or too high) and how soil pH will affect that. For most of us, this may involve occasionally adding one of a few nutrients (most commonly nitrogen), or perhaps acidifying the soil to improve nutrient availability.
How do you know when to add nitrogen to established landscape plants? Let your foliage do the talking. If leaves are uniformly yellow, small and sparse, you might have a nitrogen deficiency. This will be most common in the mid to late summer, when plants are growing most rapidly and competing with one another for resources. Be sure this symptom is wide-spread, however. If it’s just one plant showing deficiency symptoms, it’s probably not a landscape issue.