Most gardening experts agree that a late fertilizer application is a good idea in cold-weather climates where winters are a period of dormancy. A mild feeding of ornamental garden beds or vegetable gardens can also replenish soil that has been heavily depleted by growing plants over the season. While lawn care companies and manufacturers of granular and liquid fertilizers may recommend more fertilization than is necessary for a healthy garden, independent scientific studies from university programs do confirm that late summer and fall are especially good times to fertilize turf lawns.
Why Fall Is a Good Time to Fertilize
Fall is the time when cool-season grasses recover from summer stresses such as drought, heat, and disease. If the lawn has been properly fertilized in the late summer and fall, turfgrass can begin to store carbohydrate reserves in the stems, rhizomes, and stolons. These carbohydrate reserves help grass resist winter injury and disease, and serve as a source of energy for root and shoot growth the following spring. Late fall fertilization will also provide better winter color, enhanced spring green-up and increased rooting.
When to Fertilize
Although the exact timing can vary due to weather conditions and climate zone, the final fertilizer application should be made sometime in November in most regions, at the point when the grass has stopped growing or has slowed down to the point of not needing to be mowed. Do not wait until the ground freezes, however. Ideally, there is still active growth occurring, but not enough to warrant mowing.
Proper timing is essential. If fertilizer is applied too early while grass or garden plants are vigorously growing, it can invite winter injury and snow mold the following spring. Do not ever apply fertilizer to frozen soil or over snow or ice.
How Much Fertilizer to Apply
Nitrogen is the most important nutrient for a fall fertilizer. A recommended dose for lawns is one pound of soluble nitrogen for every 1000 square feet or one and a half to two pounds of slow-release nitrogen for every 1000 square feet. A complete fertilizer with a high ratio of both nitrogen and potassium (K) is essential for enhanced rooting, cold hardiness, disease resistance, and wear tolerance. Be wary of applying a fertilizer with too much phosphorus (P), since run-off of this nutrient can be very damaging to rivers and streams.
Choose the Right Fertilizer Mix
An ideal fall fertilizer blend has an N:P:K (nitrogen to phosphorus to potassium) ratio of 24-4-12 with IBDU (Isobutylidene diurea). In this formulation, a small amount of nitrogen is immediately available to the plant while the rest is in slow-release form, allowing it to slowly break down and provide an extended feeding to the grass.
The recommendations for flower and vegetable gardens are similar. A mild fertilizer feeding in the fall will replenish the soil and prepare it for a quicker green-up when planting begins the following spring. Gardens do better with this approach than with a heavy dose of fertilizer in the early spring.
Many university studies have concluded that most homeowners overfertilize their lawns and gardens. Too much nitrogen can be as damaging to plants as too little, and using natural sources of nutrients, such as compost on the garden or mulching lawn clippings rather than bagging them, can replace some of the traditional chemical fertilizer applications. Most studies now conclude that one late- to mid-summer feeding of a lawn, followed by a light fall feeding, produces a better lawn than the old recommendation for three or four major feedings for each growing season, as championed by fertilizer manufacturers.
Flower or vegetable gardens similarly can thrive with fewer fertilizer applications than once believed, especially if they are properly amended with compost and other natural organic materials. Most gardens do very well with one feeding shortly after planting and one as the growing season concludes, although plants that produce large quantities of vegetables or very large, plentiful flowers may need more.