A Little Goes a Long Way

As snow starts to melt and temperatures slowly rise above freezing, we all start chomping at the bit to start production. Obviously, early spring contracts bring much needed capital and get workers out of the shop. Most professional lawn care companies opt for spring fertilization in some form, but moderation is the key.

The benefit of spring nitrogen is to provide a quick green-up and increase customer satisfaction. However, there is almost no agronomic benefit of early spring-applied nitrogen to the turf plant except, perhaps, on starved or very neglected lawns. Conversely, there are many potential disadvantages to early spring nitrogen. Early spring nitrogen encourages shoot growth at the expense of root growth, creating shallow-rooted turf for summer. Moreover, increased shoot growth depletes energy reserves that the plant may need during a stressful summer or on lawns that will not receive irrigation. Furthermore, the increased shoot growth from early spring nitrogen increases mowing requirements and likely increases scalping on infrequently mowed lawns, which also depletes energy reserves for summer. This is especially problematic on lawns that are mowed by professionals on a weekly schedule during the spring. The bottom line is that early spring nitrogen should be minimized, if not eliminated, on cool-season lawns.

In the most successful fertilization programs on cool-season turf, 60 percent or more of the annual nitrogen is applied in the fall. Generally, at least two applications are made – one in early September and the other near the final mowing. The late fall application, in particular, is critical because it increases root growth and energy storage, creating a healthy plant for next summer. This application should be applied near the final mowing and it must be applied while the plant is still green and photosynthesizing.

If this application is missed, it cannot be made up by a winter application or an early spring application. So, applying nitrogen to dormant lawns wastes time and money and is environmentally irresponsible. The late fall application will largely determine how you should fertilize the following spring. Late fall nitrogen will enhance spring green-up tremendously, which makes an early spring nitrogen application unneeded or, if it is applied, only marginally effective for speeding green-up. Therefore, no early spring nitrogen is needed on late fall fertilized lawns.

WHERE IS EARLY SPRING NITROGEN JUSTIFIED? Early spring nitrogen can be justified on lawns where no late fall nitrogen was applied and 0.5 pounds of fast-release nitrogen per 1,000 square feet should improve green-up. Neglected, extremely thin, winter-damaged or newly seeded lawns may benefit from small doses of nitrogen starting in early spring and continuing into early summer. Frequent applications of low rates (around 0.5 pounds of nitrogen per 1,000 square feet) every four weeks in early spring through early summer may help the turf fill in. Realize, though, that this fertilization schedule will also likely increase shoot growth and, thus, mowing frequency.

The next situation that justifies early spring nitrogen is on a lawn where crabgrass is a threat and preemergence herbicides are required. Preemergence herbicides for crabgrass must be applied early in spring, shortly after turf greens up and is in the middle of the spring growth flush (see Weed Out Weeds on page 138). Since granular preemergence herbicides are almost always formulated on a fertilizer carrier, applying a preemergence herbicide virtually mandates that fertilizer also will be applied. Select a fertilizer/herbicide combination containing a low percentage of nitrogen so only 0.5 pounds of nitrogen per 1,000 square feet or less is applied. An alternative to this is selecting a product with a high percentage of slow release nitrogen to delay the nitrogen release until later in the spring when it has fewer potentially detrimental effects.

STARTING SPRING FERTILIZATION. Nitrogen application can be picked up later in the spring after the primary growth flush. Nitrogen later in the spring can help extend green color into summer and minimize low-nitrogen diseases like red thread, pink patch and dollar spot. These diseases are especially problematic on perennial ryegrass, which has become very popular in contractor seeding mixes. If you maintain a lot of perennial ryegrass lawns in the relatively warm humid areas of the Midwest and Mid-Atlantic regions, late spring nitrogen could help to minimize these diseases.

Late spring nitrogen applications should be applied after the primary growth flush of cool-season turfgrasses. Generally 0.75 to 1 pound of nitrogen per 1,000 square feet can be applied using a fertilizer with both slow and fast release nitrogen in early to late May depending the region.

There are many situations that require slightly different fertilization techniques. For instance, if you applied the preemergence herbicide with nitrogen in early spring, less nitrogen should be applied later in the spring. Or if you are making frequent applications to the lawn, lower rates could be used with little or no slow-release nitrogen. If clippings are bagged on a specific lawn, the nitrogen rate should be increased by up to 25 percent. The take-home message is that late spring fertilizer is beneficial to cool-season turf, but the exact fertilization regimen can be slightly different on each lawn.

NUTRIENTS BESIDE NITROGEN. There are a few nutrients that can successfully be used in the spring. Iron at low rates (around 0.025 pounds actual iron per 1,000 square feet) will deliver extremely quick green-up of cool-season turf plants without increasing shoot growth. Iron can be useful when dealing with clients who require the fastest green-up on the block. However, iron often becomes limiting in the spring on high pH soils when air temperatures tend to warm up quickly, but cool, wet soils limit iron uptake. Symptoms of this are mottled patches of chlorotic turf and if you look closer, the younger actively growing leaves are yellow. LCOs can compensate for this by applying a low rate of iron. Iron also can be used throughout the summer to maintain color without encouraging growth, but it should be used as a supplement to regular nitrogen applications and not as a replacement for nitrogen. But be careful with iron applications because they can stain sidewalks, siding, decks, etc.

Spring also is a great time to apply phosphorus- and potassium-containing fertilizers, though these fertilizers tend to have elevated burn potential (especially potassium-containing fertilizers). Cool spring temperatures and frequent rains should limit potential burn. Because grass plants contain about 4 percent nitrogen, 1 percent phosphorus and 2 percent potassium, we often recommend applying four parts nitrogen, one part phosphorus and two parts potassium over the course of the year. However, this is a drastic oversimplification and could lead to applying too much phosphorus and/or potassium. Soil tests are still the best and only way to determine needs of phosphorus and potassium.

Nitrogen applications should be minimized in the early spring, but can be applied judiciously after the spring growth flush is over. If you must apply early spring nitrogen, try to apply it at rates less than 0.5 pounds per 1,000 square feet.

The author is associate professor and extension turfgrass specialist, Department of Agronomy, Purdue University, West Lafayette, Ind.,
and can be reached at 765/494-9737 or zreicher@purdue.edu.