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 this article we discuss winter-kill, what it is, how it occurs, and how to detect the amount of winter-kill 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 nonprotected, low-cut bermudagrass golf course putting greens is relatively common in Oklahoma. However, moderate to severe winter-kill of bermudagrass lawns 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. It is not too early for homeowners to begin scouting for winter-kill damage.
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.
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”). 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 only available only as sod or sprigs (not seed).
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 occur. Allow seven 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 to use to gain insights into whether moderate to severe winter-kill of a bermudagrass stand has occurred. These techniques include:
•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.
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 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. If no rhizomes have survived, the stand will not regenerate.