David M. Kopec, Extension Turfgrass Specialist
In Arizona, May is already mid-summer in Yuma and Phoenix, the beginning of hot weather in Sierra Vista and is nice "long-shirt" weather in Flagstaff. At this time, the following activities should be correctly considered for lawn maintenance items.
Phoenix - Yuma - Tucson (low elevation deserts)
Many homeowners are chopping at the bit to get rid of the ryegrass winter lawns and have a rapid return (forced transition) to bermudagrass. There are as many "home remedies" on how to do this transition, and some of them can involve some drastic measures.
Scalping the lawn:
Current research at the University of Arizona's Karsten Turfgrass Facility shows that scalping the lawn once or twice a week is not beneficial for getting rid of the ryegrass quickly. It weakens common bermudagrass and is detrimental to it. For overseeded common bermudagrass (normally mowed at 1.5 inches), the best transition (most bermudagrass coming back) is occurring when the mowing height is maintained at the regular height (unscalped).
All things equal, Tifway (419) and Santa Ana are more aggressive than common bermuda and many of the other new seeded types. Transition is usually less of a problem for these grasses. I do not know if the "scalping" treatment on Tifway (419) is more successful (for transition) than that of common bermudagrass.
Shutting Off The Water
This is a trick to kill the ryegrass, but it also damages the bermudagrass. The bermudagrass has been irrigated since last November on a continuous basis, since it is the live "undercover" for the ryegrass overseed.
Drying out the stolons, crowns and shallow rhizomes can severely damage the bermudagrass. Yes, it will come back, but the homeowner ends up severely over watering and fertilizing to bring back the bermudagrass. This is not a good idea.
Verticutting, Dethatching and Reno-thinning
These are synonymous terms for de-nuding the canopy by vertical mowing. This damages the ryegrass by cutting into the growing point (crown). Since ryegrass is a bunch grass (reproduces by tillers) it does not have stolons or underground rhizomes to grow back from. When you injure the ryegrass crowns, you will also cut stolons off the bermudagrass (at minimum). If you verticut deeply (as homeowners may do) then crowns and rhizomes can be damaged. A "shallow" verticutting in three directions is much better than beating the lawn into submission, for transition sake.
Verticutting can be done after the night time low temperatures reach 60F for seven days in a row. After verticutting, follow with 1.0 to 1.5 lbs. of actual nitrogen per thousand square feet. Continue a normal irrigation schedule after the fertilizer is watered in.
The same rules apply to Sierra Vista, Globe and Safford. You just have to wait longer until the night time low temperatures are 60F!
Show Low - Payson - Flagstaff (high elevations)
May is the time to apply the last spring fertilization to the lawn. The spring schedule should apply only 1/3 of the yearly amount with the other 2/3 being applied in the fall. Over fertilizing cool season lawns (Kentucky bluegrass, ryegrass, fescue) is disastrous for the grass. Too much nitrogen causes the existing shoots to elongate fast, use up food reserves quickly (needed for summer stress) and causes scalping and a shallow root system. More on this in the next issue. In May, apply 1/4 to 3/8 of a lb./1000 ft. of actual nitrogen per square foot of lawn. The product can either be a -N- only source (ammonium nitrate) or come from a complete fertilizer which has N-P-K. Examples include 10-6-4, 21-7-14, 20-10-10, etc. The product can also contain iron and/or sulphur, as well.
In the heat of the summer, apply iron instead of nitrogen for maintaining green color, and raise the mowing heights.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.