Summer Mowing and Fertilization – Turfgrass Science at Purdue University

Summer Mowing and Fertilization

By the time you read these words the longest day of the year has approached and the official calendar “Summer” season will have begun. Just as the weather changes as we transition between seasons from late-Spring to Summer the turfgrass plant changes too. First, both air and soil temperatures rise beyond what is optimal for cool-season grass shoot and root growth. Additionally, the elevated humidity affects the plant. The combination of high soil and air temperatures combined with elevated humidity can create a growing environment that is potentially stressful for many cool-season grasses throughout the region. This is primarily due to the fact that during the summer cool-season grasses do not manufacture food very efficiently and begin to rely on their stored carbohydrates to sustain them through the stressful summer growing conditions. Additionally, the added humidity when combined with excess rainfall or improper excess irrigation makes the environment favorable for many fungal pathogens and hence more disease damage is also observed. 

So what can you do to promote a healthy turf during this period?

To start with it is always a good idea to carefully evaluate your primary cultural practices, specifically mowing and fertilization. Mowing: For lawn turf areas you should be mowing as high as practically possible, meaning 3+ inches or almost as high as many commercial mowers will allow. This will provide a greater leaf surface area for the manufacture of plant foods while also promoting the deepest possible rooting. From a practical side it also allows for longer intervals between mowing events will enable you to recycle clippings back into the turf more easily (which also allows you to recycle valuable nutrients back to the turf). Lastly, this is a great time to consider sharpening your mower blades. Sharp mower blades reduce turf stress (mowing does wound the plant afterall…), take less energy to cut the leaf blade and may reduce fuel consumption when mowing dense turf areas.

You may begin to notice that with the onset of Summer that the growth of most lawn grasses begins to slow. This is somewhat of a plant response to the unfavorable growing environment, but in a year like this one may also be due to a depletion in soil nutrients. In West Lafayette we have had one of the wettest April and May periods on record. Fertilization: Although these rains may have been beneficial in terms of active growth, these heavy rains likely pushed some nitrogen and potassium through the soil as well. Thus, some lawns are beginning to loose the dark green color from early Spring. If that dark green appearance is important to you, then a modest fertilization may be appropriate. By contrast, heavy Summer fertilization is very risky. Fertilizer source and rate are the most important considerations. To boost color and promote growth, a good starting point would be to apply ½ pound of actual nitrogen per 1000 ft2. Next comes fertilizer source selection…. Avoid fertilizer products with high (50% or more) amounts of soluble or quick release nitrogen. Although these sources are great where fast greening is desired, they also promote rapid shoot growth at a time when the plant is trying to conserve energy.  Some suggestions for fertilizing at this time of year include products with high amounts of methylene urea, or many of the natural organic sources. If you have difficulty calculating how much fertilizer you need you can always use the Purdue Turf Science fertilizer calculator. The additional benefits of modest summer nitrogen are that the increased turf vigor may allow for faster recovery to any occurrence of turf diseases.

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