Low Temperatures in winter of 2013/2014 Bring Possibility of Increased Winter Injury to Bermudagrass Stands

Justin Quetone Moss, Assistant Professor, and Dennis L. Martin, Professor, Turfgrass Science Program - Oklahoma State University

Due to the cold temperatures throughout Oklahoma this past December and January, some areas have experienced more winter-kill of bermudagrasses than in most previous years. Winter-kill is a relative term, meaning that some portion of a plant or portion of a turfgrass stand has died during the winter. In the areas that the authors have scouted thus far, the injury is worse than average, but less severe than that found in spring of 2010 and certainly no where near the damage from the catastrophic winter of 1989/1990 (the Gold Standard for Bermudagrass Winter Injury). As expected, the worst injury is in August-seeded or August sprigged bermudagrass stands as well as on severely compacted sites such as heavily used footpaths and athletic field surfaces that were over-used. In this article we discuss winter-kill, what it is, and how to detect the amount of winter-kill prior to spring greenup so that planning can begin to effectively help the turfgrass stand recover in spring.

Winter-kill or tissue death during the winter, can be from dehydration, true low temperature injury, or a combination of the two. For the purposes of this article, we discuss winter-kill associated with low temperature injury. Cold temperatures can damage warm-season grasses, such as bermudagrass, through a series of days and nights with sustained below freezing temperatures or a series of unseasonably warm days followed by a sudden extreme drop in temperature to well below freezing. The grasses are especially vulnerable if they are types with poor winter tolerance and they are left unprotected by some type of cover, such as snow, straw, or geotextile tarp, and when soil temperatures are also below freezing.

Moderate to severe winter-kill of non-protected, low-cut bermudagrass golf course putting greens is relatively common in Oklahoma. However, moderate to severe winter-kill of bermudagrass lawns, sports fields or golf course fairways is a fairly rare event. The amount of damage present on any bermudagrass stand varies greatly from year to year due to differences in weather and how a turfgrass stand has been managed.

Cases of winter-kill can be severe, such as when an entire turfgrass stand dies and no plant parts survive to regenerate the stand in spring. However, during most Oklahoma winters, only small portions of the upper aerial shoots system are killed. In the latter case, most crowns (growing points) located in the lower canopy, survive, leading to rapid greenup and turfgrass stand regeneration in the warm days of mid to late spring. Winter is not yet over in Oklahoma so we cannot project with great certainty the amount of winter-kill that will be detected once the time of spring greenup arrives. However, it is not too early for homeowners and professional turf managers to begin scouting for winter-kill to assess the damage that has already occurred thus far.

Areas that are most likely to experience winter-kill are:

•       Areas with heavy foot or vehicle traffic.

•       North facing hills and slopes.

•       Areas with moderate to heavy shade, including areas that receive adequate sunlight in summer but that remain in continuous shade from mid-September

                   through mid-March.

•       Areas with poor drainage.

•       Areas with low soil fertility.

•       Areas with excessively high fertility or that received high rates of fertilizer late into the previous growing season.

•       Areas that are thin or that were planted much later in the growing season and did not completely cover prior to the first frost.

•       Areas with less cold tolerant bermudagrass cultivars.

Bermudagrasses known to have suffered greater amounts of winter-kill as assessed in multiple past research trials include ‘Arizona Common’, ‘NuMex Sahara’, ‘Sahara’, ‘Tifway’ (also known as ‘419’), ‘Mohawk’, ‘Princess 77’, ‘Celebration’, and ‘Sultan’. Most common bermudagrasses sold as ‘U-3’ have demonstrated respectable levels of winter tolerance although there can be great variability in cold hardiness of types sold as U-3.

There are several bermudagrasses that have improved winter-hardiness based on research trials conducted around the United States. Cultivars that have both high visual quality and improved winter tolerance that were developed at Oklahoma State University include ‘Riviera’ and ‘Yukon’, both available as seed, as well as ‘Patriot’, ‘Latitude 36’, and ‘NorthBridge’, which are interspecific hybrids and available only as sod or sprigs (not seed). Other bermudagrasses that also have improved winter tolerance are ‘Hollywood’, available as seed from Jacklin-Simplot, and older, less easily found vegetatively propagated bermudagrasses such as ‘Midiron’, ‘Midfield’,‘Midlawn’ and ‘Quickstand Common.’ The bermudagrass called 'Astro', readily available in the Oklahoma sod trade, has also demonstrated good winter tolerance based on our field trials at Oklahoma State University.

When air temperatures have fallen into the low teens or several days have elapsed when air temperatures never rose above freezing, scouting for winter-kill of bermudagrass should be undertaken to examine the impact of the freeze events. Allow 7 to 10 days to elapse following a severe winter event to assess its effect on the bermudagrass stand. Several techniques are available for the homeowner or professional to use to gain insights into whether moderate to severe winter-kill of a bermudagrass stand has occurred. These techniques include:

The "Window sill assessment" or "plug collection and greenup assessment technique"

Use a soil probe or shovel to collect a bermudagrass plug that is at least 4-6 inches deep and at least 3-4 inches in diameter. If possible, collect multiple plugs and include plugs from both an expected good area and bad area. Label the plugs/pots accordingly. Place the plugs in a pot, water as needed, and put the pots in a warm (room temperature or above), sunny area such as a south facing window. If needed, provide supplemental lighting to ensure the plugs receives adequate light for at least 8 to 10 hours per day. Monitor the plugs for 7-21 days for bermudagrass green-up and growth. If there is no growth on the plugs by 21 days, it is likely that the area has experienced severe winter-kill.

"Canopy brushing technique"

This technique is useful in “real time” in the field and can help to locate surviving aerial stems and shoots. Use a leather glove to protect hands.    Vigorously bush and defoliate (remove leaf blades) on 6-12 inch diameter turf sampling areas. Sample multiple areas, including suspected good and bad areas. Assess the density of living aerial shoots that show green, red, purple, or white segments on the stem between the leaves (internodes) in the lower canopy. The stems often feel sturdy or rigid and may somewhat “snap” when bent in two. If no green, red, purple or white (not tan) color is seen on above ground stems or if they look brown, black, tan, or feel very soft or flimsy, then winter-kill has likely occurred to the entire aerial shoot system. Next proceed to assessing survival of the below ground shoot system, including the rhizomes.

"Combination technique"

After canopy brushing, plugs can also be removed from the area to check for survival of underground stems including the rhizomes. Rhizomes are generally white, horizontal growing stems with unusual white leaves that have extremely short leaf blades, no leaf sheaths and a sharp growing point at the terminal shoot end. After collecting the plugs, break up the plug to locate the rhizomes. They will look noticeably thicker and /or larger than roots. Look for white, firm rhizomes. Similar to the aerial shoot technique, the rhizomes will feel sturdy or rigid and may somewhat “snap” like a garden-fresh vegetable when bent in two. If the rhizomes look brown, tan, or black and if they feel very soft, flimsy, or mushy, then winter kill of that particular tissue has likely occurred. Rhizomes are the surviving shoots of last resort. If no rhizomes have survived, the stand will not regenerate. Even if some rhizomes survive, if complete aerial system kill has occurred, the homeowner or professional manager may not be willing to wait for the rhizomes system to regenerate the turfgrass stand. Fortunately complete kill of the subterranean shoot system (including rhizomes) is rare on areas other than putting greens in Oklahoma.

It’s best to scout for winter-kill in advance of spring greenup and prior to application of the summer annual pre-emergent program. Most pre-emergent herbicides inhibit root formation on stolons to some degree and thus slow recovery of a stand that is being grown-in after severe winter-kill. Thus, if substantial winter-kill of bermudagrass has occurred, an informed decision as to whether to use normal use rates, reduced use rates or even forego application of the normal first application of the pre-emergent herbicide must be made by the homeowner or site manager. If substantial amounts of winter-kill have occurred such that the stand is not filling in quickly in spring, visit our website at http://turf.okstate.edu for information about re-planting or renovation (see Fact Sheet HLA-6419 – Establishing a Lawn in Oklahoma).