The Best Fertilizer for Fall

While some manufacturers insist that plants need fertilizer to be properly "winterized," gardeners focus on the specific kinds of growth fertilizers support during the dormancy of winter. The goals of fertilizing shrubs, trees, bulbs and lawns late in the growing season are to strengthen them for winter cold and to create a reserve of nutrients ready for when growth resumes in the spring. You may find some of the fertilizers recommended by professionals surprising.

Fertilizer contents are noted on the package in a three-number ratio, such as 10-10-10 or 5-10-5. This is known as the N-P-K ratio, telling you the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K) that it contains. Nitrogen is essential to leaf formation, and the high percentage of nitrogen in most lawn fertilizers (for example, 25-5-5) is the source of that great green spread. Phosphorus is needed for flower and fruit development, and both perennial and vegetable garden fertilizers usually contain a generous percentage of phosphorus (10-10-10 or 5-10-5). Potassium governs overall cell development and is part of any balanced fertilizer formula. A fertilizer label will also indicate the presence and percentages of micronutrients such as boron, copper and calcium.

Although logic might dictate providing a high-phosphorus fertilizer to support root growth during the winter, grass experts do not agree on that approach. According to garden writer Roger Cook, a high-phosphorus fertilizer can be applied effectively in September, but the critical fall feeding, usually applied two to three weeks before the ground freezes, contains a high level of nitrogen. Some of it is absorbed immediately, healing summer heat and drought damage to plants, while the rest remains in the soil to be used for new growth in early spring. Tests of high-phosphorus/high-potassium "winterizing" fertilizers have been conducted only on warm-season grass varieties such as Bermuda and Zoysia. According to Colorado State University Extension lawn experts, there is no evidence supporting the benefits of winterizing fertilizers for cold-season grasses such as bluegrass, ryegrass or fescue. Granular fertilizer lasts longer than liquids, and a fertilizer containing slow-release nitrogen, usually listed as ureaform, sulfur-coated urea, IBDU or Milorganite, will be most effective.

For woody shrubs, experts may debate about whether a high-nitrogen or more evenly balanced formula is best, but there is general agreement on the old feeding maxim "late and light." A small quantity of granular fertilizer as a side dressing, at the edge of the branch canopy, can be applied late in October or early in November, roughly three weeks before the first hard frost in your area. The goal is to provide a reservoir of fertilizer to support early spring growth, rather than stimulate fall growth in the instance of an unpredicted late warm spell. As woody shrubs age, their need for nitrogen decreases, and many mature shrubs need only a spring feeding.

"Late and light" applies to trees as well as shrubs. The bulk of tree roots grow in the top 18 inches of soil, no matter how large the tree, with 50 percent of them in the top 6 inches of soil for some varieties. Cultivating around roots can disturb them and make them more vulnerable to chilling temperatures. Spreading a light layer of granular fertilizer at the base of the canopy encourages absorption by the widespread small roots that feed the tree. Like shrubs, trees gain more benefit from spring than fall feeding.

Fall-planted bulbs benefit from a teaspoon of fertilizer in each planting hole. Bone meal, a good source of calcium, is the traditional food. In areas where bone meal attracts large numbers of digging rodents like squirrels, some gardeners favor superphosphate instead for strong root development.