When to Apply Fertilizer and What Kind Explained

Q. When during the year should you start and stop fertilizing landscape plants and what kind of fertilizer is best for them all? It seems to me that with acid loving plants, cacti, palms, roses, fruit trees and annual flowers they might all require different kinds of fertilizers and different times to apply them.

A. You could go crazy trying to follow all the different rules when fertilizing for different types of plants. Keep it simple. Let me give you a few simple rules to follow when applying fertilizers.

This is an easy fertilizer for homeowners because it tells them what it's for. It has 10% nitrogen, 5% phosphorus in the form of P205 and 10% potassium in the form of K2O. But it should really tell us that it's primarily for frond and stem growth (nitrogen), half as much for roots and flower production (phosphorus, we don't want much of that anyway) and a similar amount of potassium as nitrogen. What other plants would this be good for besides palms?

            If plants are winter tender, in other words they might get hurt or die when temperatures dip below freezing, stop fertilizing these plants in July. Our citrus trees fall into this category.

            Lawns, bedding plants, such as annual flowers, and vegetables should be lightly fertilized once a month. Lawns that are expected to remain dark green during the winter should have fertilizer applied around Thanksgiving before freezing weather.

This is 21% nitrogen in the form of ammonium sulfate. There is no number for on this label but it also delivers about 20% sulfur as a fertilizer as well. Some people are very negative about ammonium sulfate but the plant doesn't really care where the nitrogen is coming from. We should pay attention to any contaminants, such as heavy metals, that might be in this bag of fertilizer.

            For light fertilizer applications, reduce the amount applied to half the rate recommended on the bag or container. Light applications of fertilizer can be applied every month and immediately watered in if applied early in the morning. Get in the habit of applying fertilizers early in the morning or late in the day.

            The most highly prized landscape plants should be fertilized three or four times during the year; January/February, April/May and September/October. These include plants like roses, gardenias, and Jasmine for instance. Again, use half rates when applying fertilizers.

Has a lot of phosphorus in it. The numbers tell us that. This fertilizer would be used for new plants that need to create a lot of new roots and those that flower or produce seed. Marijuana producers use this type of fertilizer when the plant is getting older and is close to flowering and producing seeds.

            Most landscape plants are fertilized only once, just before new growth begins in late January or early February. This includes all landscape trees including palm trees.

            Which fertilizer to use? You can get by with 2 or 3 fertilizers in your arsenal. That’s all. Fertilizers have three numbers separated by hyphens somewhere on their label. They represent three different plant nutrients; nitrogen, phosphorus and potassium and in that order.

            When growing plants that are primarily important because of their leaves and stems, the first number, or nitrogen, should be the highest. The middle number, phosphorus, should be about one fourth of the value of the first number. The last number, or potassium should be somewhere in between the first and second number.

Obviously then when growing roses, fertilizers with a high middle number are important for good flower production. Be careful of adding high phosphorus fertilizers over and over because phosphorus will begin to accumulate in soils unlike nitrogen and potassium.

            When growing plants valued for their flowers or fruit, then the second number or phosphorus becomes critical. It needs to be the highest. When fertilizing these plants, the second number should be highest while the first and third numbers lower. Exact numbers are not critical but the ratio of these three, or their proportions contained in the fertilizer, is more important

            To be healthy, plants need more nutrients than supplied by only these three numbers. But these three numbers represent nutrients needed in massive amounts by plants. The other important nutrients are supplied by the soil. For this reason, I frequently mention the application of compost. A compost application, once a year to landscape plants, would be extremely beneficial.