Longer days and warming temperatures accompany a renewed interest in the lawn and landscape, and one of the first things most homeowners desire is a lush, dark green lawn. This is most often achieved by way of spring fertilization, but how much fertilizer is appropriate in balancing the aesthetics of the lawn versus overall turf health (and possibly even environmental concerns)?
What is unique about spring turf fertilization in Virginia?
This is a transition zone state in which all grasses can be grown, just not always very well! In short, our likely extremes in climate during the winter and summer months result in environments that are very stressful to warm and cool-season grasses alike. Hence, we need to constantly prepare our grasses for the next environmental stress period that is approaching.
Understanding your turf
While we get the biggest visual reward from maximizing the shoot system (i.e. maximum numbers of lush, dark green leaves and stems), the foundation of a healthy lawn lies in having the most roots and carbohydrates possible in order to deal with the pending stress periods. In short, anything that can be done to maximize the root system and carbohydrate reserves improves the health and overall fitness of the plant.
Spring fertility strategies on cool-season turfgrasses
The figures presented in this and the following section detail the anticipated seasonal changes in root, shoot, and carbohydrate (i.e. food) production levels in cool and warm-season grasses. For cool-season turfgrasses, there is an increase in root production in the spring as soil temperatures begin to warm. This is quickly followed by a major surge in shoot growth as the plant accelerates its photosynthetic capacity. From a practical standpoint, a total nitrogen (N) application rate of 0.5 to 1 lb N/1000 sq ft during the late winter to mid-spring months can benefit root development and enhance spring greening. The key to success is in keeping away from the "if a little is good, more is better" philosophy. Ideally, the majority of spring root growth should be promoted by an aggressive fall N fertilization program from a few months earlier; using this approach, the N for spring root growth is already inside the plant and is quickly mobilized and metabolized to contribute to the newly forming roots. It is recommended that approximately 3/4 of the seasonal N requirement for a cool-season grass be applied in the fall. For instance, in a high maintenance Kentucky bluegrass lawn that receives up to 4 lbs N/1000 sq ft/year, 3 lbs of N would (should!) be applied in the September to mid-December period. Of course, that much N is not required (nor desired) for all situations.
Fertilizer source obviously matters as well. For water soluble sources, it is strongly recommended that you apply no more than 0.5 lb N/1000 sq ft per application in order to maximize N-use efficiency and minimize nutrient leaching and run-off potential.
Spring fertility strategies on warm-season turfgrasses
Though vastly different grasses, the fertility requirements for the spring are almost identical: a little goes a long way! As figure 2 indicates, root production and carbohydrate levels sharply drop as almost all reserves are utilized to promote the development of shoots for photosynthesis. If possible do not initiate spring N fertilization until after complete greening of the shoots. Spring root growth of most warm-season grasses is quite limited until soils warm significantly and, and heavy spring N fertilization (> 1 lb N/1000 sq ft) can further decrease the root:shoot ratios. Upon complete greening, a regular fertility program supplying up to 1 lb N/1000 sq ft per growing month can be employed for the duration of the warm-season growing period IF needed. Again, this depends on the grass and the level of maintenance desired or available: bermudagrass and St. Augustinegrass might receive up to 4 lbs N/1000 sq ft annually, whereas grasses like centipedegrass and zoysiagrass should receive no more than 2 lbs N/1000 sq ft annually.
Other nutritional or lime needs?
The only way to clearly understand what additional modifications your soil needs to optimize turf performance is to soil test for levels of pH, phosphorus (P), and potassium (K). The one nutrient that requires all turf managers (homeowner to professional) to give particular attention to is phosphorus. Many of Virginia's soils that have been regularly fertilized over the years have more than adequate P according to soil tests and no further applications of P are required and/or desired. Phosphorus is known to be a major contributor in water quality problems and the likelihood of non-target effects on water quality due to leaching into groundwater etc. are exaggerated when P is applied to soils that test High to Very High in soil P. When P is needed, a soil test will recommend an appropriate fertilizer and application level, but the use of traditional complete garden-type fertilizers (e.g. 10-10-10) without the benefit of a soil test is not representative of environmental stewardship.
Where does all of this fit with commercially available "Weed and Feed" materials and programs?
These products are often the first part of a 4-step program touting a great looking lawn. There are several well respected companies that market such programs. The ability to apply fertilizer and PRE weed control in a single application is certainly attractive in this first part of the program, but there are still some things to consider in choosing the convenience of these programs. First of all, since most of the fertilizer carriers of the PRE herbicide contain 70-80% water soluble nitrogen and 20-30% water insoluble nitrogen, there is going to be a significant growth and greening response with the product's application. At the recommended use rate for many of these materials, they might be supplying up to 1 pound of N/1000 sq ft. For cool-season grasses, this is not an unrealistic value IF this is the only application made in the spring in our transition zone climate (i.e. steps 2 and 3 in the program that might also be delivering 1 lb N/1000 sq ft for late spring and summer applications are not recommended). The application timing on warm-season grasses for Virginia is also somewhat troubling because the treatment is almost always made before or during winter transition. The N would be much more beneficial to both roots and shoots IF it was applied after complete spring greening. However, in order to gain PRE weed control, the chemical has to be applied prior to greening. Certain parts of the 4-step programs simply do not fit optimal transition zone fertility management programs for either cool- or warm-season grasses.
More discussion on this and other topics is available in VCE publications 430-522 (Spring and Summer Lawn Management Considerations for Cool-Season Turfgrasses) and 430-533 (Spring and Summer Lawn Management Considerations for Warm-Season Turfgrasses).
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