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ATA Turf Times – Dave Han, Ph.D. Auburn University and Alabama Extension

Anyone who has ever managed turfgrass, whether in their own lawn or professionally, will have lots of questions. In my experience, the word that pops up most frequently in questions about turf management is “when.” We’ve all asked or been asked: When should I fertilize? When do I put out my preemergence herbicide? When is the right time to sprig / seed / sod? When should I take a soil test? When should I irrigate? The list goes on.

With so many questions related to timing, calendars are a very popular product to remind people when to do certain things. Calendars can be very useful, but they are not foolproof. Mother Nature does not always provide weather that fits the calendar date, and since so many decisions about turf management are driven by temperature, rainfall and humidity (or should be), there is no substitute for looking at actual thermometers and rain gauges before making decisions. But what I think can be more valuable is a general seasonal guideline of what to expect, what to look out for, and how to plan so when a particular season comes around, turf managers are making good decisions on a monthly or weekly time scale.

Since the Alabama turfscape is dominated by warm-season grasses, this article will focus on those.

SPRING

Spring is usually the hardest season to deal with because it brings unpredictable weather. Wild temperature swings, severe weather outbreaks, and a human tendency to want to rush things can all lead to making some bad calls when it comes to spring turf management. Spring is the season when warm season grasses begin their growth after dormancy or semi-dormancy in the winter, when summer annual weeds begin to germinate, and when winter annual weeds are their most noticeable and troublesome. It is also when plans should be made to plant sprigs or seed of warm season grasses.

The main factors to consider in the spring are:

• When to apply postemergence herbicides to control winter annual weeds
• When to apply preemergence herbicides for summer annual weed control
• When to begin fertilizing the turfgrass

All of these decisions will be influenced heavily by air and, especially, soil temperatures. Table 1 provides an overview of some important soil temperatures that affect management decisions. Timing becomes tricky, as soil temperatures will vary wildly from year to year, and even swing up and down within weeks during any given spring. What happened last year may not have much to do with the appropriate timings for this year’s management decisions, especially when it comes to herbicide treatments.

Table 1. Key 4-inch soil temperatures for turfgrass management decisions

Sources: Turgeon & Kaminski (2019), McCarty (2011), Turgeon (2004)

Preemergence herbicides should ideally be made before the conditions are right for weed seed germination. Some years, the soil will warm quickly and applications will need to be made in January. Other years, they may wait until late February. Also, since there is such a variety of soil temperatures at which various weeds germinate, multiple applications of preemergents may be needed depending on which weeds are problems for any given turf.

Applying postemergence herbicides can be done very early in the spring, providing that the air temperature at the time of application is above 55 degrees F. When it is cooler, the efficacy of postemergence herbicides is greatly reduced. Special caution should be used when the warm season grass is in the middle of greenup, just coming out of dormancy. Many warm season grasses can be damaged by herbicides at this time, even though they are perfectly tolerant when actively growing. I recommend avoiding all postemergent herbicides if possible while the turf is greening up – when this happens and how long it lasts, again, varies from year to year. Let a soil thermometer be your guide; when the soil temperature hits 70 degrees F, it’s likely safe to apply postemergents as usual.

Figure 1 shows a centipedegrass lawn in Auburn, AL that was still in mid-greenup phase in the middle of May 2020, due to an extended warm, but not hot, spring that year. As temperatures warmed at the end of May, the lawn was fully green and growing by June 1.

Fertilizer (specifically nitrogen) should not be applied too early. If it is too cool to support grass growth, fertilizers will be used by weeds instead of the turf. Timing is especially important when using soluble, quick-release fertilizers. Slow release fertilizers can usually be applied at any time, as they tend to release more nitrogen when the soil is warm, which is when the grasses will use more nitrogen.

SUMMER

Once the soil temperature is consistently in the 70s F and the air temperature is hitting 90 regularly, it’s time to think about summer turf care.

The major issues with most turfgrass during the summer are:

• Irrigation: How much and how often?
• Weed control, especially of summer annual broadleaves
• How much fertilizer to apply

Most warm season grasses will use between 1 and 1.5 inches of water per week during the summer months. This does not mean that the grass will need to have 1 to 1.5 inches of irrigation applied each week. The primary aim of irrigation should be to SUPPLEMENT natural rainfall. A rain gauge is your best friend when irrigating, while a calendar is your worst enemy. In an Alabama summer, weeks may go by with no need for irrigation at all on a native soil lawn. Applying too much irrigation not only wastes water but can also promote disease and weeds/moss/algae in turfgrass.

Professional turf managers will also want to monitor the moisture content of the soil using a commercial moisture probe and meter. Golf course putting greens and athletic fields often are kept drier and firmer than a lawn for better playability, so it is important to monitor soil moisture closely. The exact water content that works best for a given green or field without causing wilting will have to be determined locally. Many times, this will be somewhere between 15%–20%, but again, this will need to be worked out for each location individually.

Many summer annual weeds can continue to germinate all summer long in very warm soil, so for controlling weeds like doveweed, spurges, chamberbitter, etc. a summer preemergence herbicide application may be necessary. These weeds can be killed post emergence too, but the continuing seed germination means that a postemergent herbicide will need to be applied monthly during the summer to keep summer annuals at bay.

Fertility requirements vary widely depending on grass species and the amount of traffic. For general lawns, the recommend rate of nitrogen to apply during the growing season is:

• Bermudagrass: 3–5 pounds N per 1000 square feet
• St. Augustinegrass: 3–4 pounds N per 1000 square feet
• Zoysiagrass: 1–2 pounds N per 1000 square feet
• Centipedegrass: 0-1 pounds N per 1000 square feet

More, sometimes much more, nitrogen will be used on golf courses and athletic fields with heavy traffic. Nitrogen should be applied at 1 pound of actual N per 1000 square feet. Be aware that higher N rates will mean more mowing. See Figure 2 for an example of how N rate affects mowing frequency in bermudagrass lawns.

Figure 2. Effect of N rate on mowing frequency of Tifway bermudagrass

Number of mowings required during the two-month periods listed to keep the grass at a height of 1.5 inches, using the 1/3 rule. Guertal and Han, 2011.

Phosphorus and potassium should be applied according to soil test recommendations. There is no substitute for a soil test to determine whether turf needs these nutrients, and how much. The same is true for lime requirements.

Fire ants are pests anytime that grass is growing, but especially in the summer. Baits are the most cost-effective way of managing fire ants, but they must be used correctly. Baits only work if they are spread while fire ants are out looking for food. If it is too hot, they may not be foraging and so will not pick up bait. Test to see if fire ant workers are active by placing a potato chip or hot dog on the grass and waiting for 10 minutes. If the food is covered in fire ants, it is a good time to spread bait.

FALL

As air temperatures and soil temperatures drop in the fall, warm-season turf managers should be thinking about:

• Preemergence applications for controlling winter weeds
• Overseeding (athletic fields in particular)
• Tapering off fertilizer application

Perhaps the most troublesome winter annual weed is annual bluegrass, Poa annua. Many times, Poa is the main winter weed and it can be difficult to control because of its wide genetic variability. The extreme variability of Poa means that there are some Poa populations that can germinate in very warm soil (upper 70s or warmer), some that can germinate in cool to cold soil (lower 50s or cooler) and some that germinate in all temperatures in between. This means that Poa germinates for a long time in the fall, winter, and early spring, so repeat applications of preemergence herbicides are often needed. When soil temperatures drop into the low 70s, it’s time to start thinking about the first preemergence application for Poa.

Another problem with Poa is that some populations have developed resistance to almost all of the modes of action that are available in preemergence herbicides. Rotating modes of action can help, but if you are cursed with resistant Poa, then plan on a lot of post emergent sprays as well.

Overseeding with ryegrass is common on athletic fields, especially fields that will see heavy use while the grass underneath is dormant, such as baseball, softball, and soccer fields. Perennial ryegrass is the main species used to overseed athletic turf, and it will germinate when the soil temperature drops into the 60s F (see Table 1). Overseed rates vary based on location and budget, but around 400–600 pounds per acre is a good baseline for an athletic field. More seed will be needed for heavily used fields. Remember to save seed to fill in damaged spots.

When overseeding, do not fertilize the ryegrass until a week or two after it is up and growing. This will ensure that the ryegrass is able to absorb the fertilizer, not the warm season grass. Ryegrass typically will need 1–2 pounds of N per thousand square feet during an overseed season. Be sure not to overirrigate while establishing overseed – keep the seeds moist but do not saturate the soil.

Fall is also a time when diseases can be an issue in warm season turf (excepting putting greens, of course, which see disease problems all the time). As the grass growth slows down, the turf is more susceptible to fungal infections. This is most noticeable in areas that do not drain well or are over irrigated. Managing water is critical to keeping diseases under control. Remember that warm season grasses need less water in the fall than in the summer. Keeping the irrigation at summer levels often causes problems.

Fertilizer applications should also taper as the soil cools. Remember that warm season grasses need soil temperatures in the 70s F for good root and rhizome growth. As the soil cools below 70, N applications should end. Potassium could still be applied if soil tests indicate a need. Fall is also a good time to apply lime, if needed, as it will react over the winter.

WINTER

Dormant turf may not need as much care as actively growing turf, but several issues recur every year:

• Winter weed control besides Poa, especially lawn burweed
• Colder than normal temperature concerns
• Warmer than normal temperature concerns

Winter broadleaf weeds include lawn burweed, which has been especially problematic in recent years. This weed germinates in cool soil (60s F) usually in the late fall, but grows all winter. It is easy to miss in the winter because of its low growth habit. It becomes a major problem in the spring after it flowers and sets fruit, because the fruits have sharp spines which injure people who step or fall on them. Winter is a great time to scout for burweed plants and kill them before they make the spiny fruits. Figure 3 shows a young lawn burweed plant as it appeared in November 2020. Most of the commercially available postemergence herbicides for broadleaf control will kill lawn burweed. Postemergence herbicides can be tank mixed with a preemergence herbicide to provide control in the late winter after a fall application has worn off.

Some years, cold weather presents a threat of winter kill. Although it is rare to have widespread winter kill on zoysiagrass or bermudagrass lawns in Alabama, it does happen occasionally. Putting greens with bermudagrass can be more affected by cold weather, and covers have been shown to prevent winter damage. Covers do have drawbacks: they are labor intensive to install and remove, and they must be removed to avoid trapping humidity and heat during the day and increasing disease pressure.

Warmer than usual temperatures cause their own problems. One major issue with warm winters is that it can cause weed seeds to germinate earlier than “normal.” This means that turf managers are caught unprepared to deal with weeds that they are not expecting to appear so early in the year. Disease issues are also more prevalent during warm winters, especially if the weather is also wetter than usual.

A seasonal mindset can help to mitigate problems that can occur when reality does not match a calendar. Knowing what to expect, and how that can change, in each season lets turf managers anticipate their needs better and provide better turf year round.

References

McCarty, L.B. 2011. Best Golf Course Management Practices, 3rd ed. Prentice Hall, River, NJ.

Turgeon, A.J. and Kaminski, J.E. 2019. Turfgrass Management, edition 1.0. Turfpath, State College, Pa.

Vargas, J.M & A.J. Turgeon. 2004. Poa annua: Physiology, Culture and Control of Annual Bluegrass. Wiley & Sons, Hoboken, NJ.