Overview.

With all of the joys that come with living in the most beautiful region of the country, we also have the frustrations of trying to grow grass in the transition zone. One of these frustrations comes from the ongoing process of trying to keep our bermudagrass fairways and athletic fields looking acceptable during those unsightly dormancy months, when the chlorophyll closes up shop and heads south for the winter. We deal with bermudagrass’ personal protest against our climate by overseeding with perennial ryegrass in the fall to keep the turf green throughout the dormant months. While Mother Nature takes care of most of the perennial ryegrass via higher temperatures and competition from bermudagrass, there are often unsightly patches of perennial ryegrass that remain throughout the summer. Researchers at Virginia Tech are continuously testing new products and finding new ways to aid in this transition to allow for quicker recovery and uniform stands of the bermudagrass during the spring and summer.

History of spring transition.

Newer perennial ryegrass cultivars are just too hardy and tend to persist throughout the year in the transition zone or die too late in the season to allow for bermudagrass growth. Cultural practices such as verticutting and fertilizing to speed up bermudagrass “greenup” don’t completely kill perennial ryegrass in the transition zone. Thus, herbicides have become a primary tool for a more rapid transition. There are now a number of products which are labeled for control of perennial ryegrass in bermudagrass, including Kerb^TM (pronamide), Manor^TM (metsulfuron), Corsair^TM (chlorsulfuron), Revolver^TM (foramsulfuron), Monument^TM (trifloxysulfuron), Tranxit^TM (rimsulfuron), and Image^TM (imazaquin). Kerb^TM, which has been labeled for spring transition for years, can be a dangerous option on uneven slopes because of its tendency to run-off to non-target areas, causing damage and death to off-site cool season grasses in watersheds. Image^TM gets little use as a transition herbicide because it has shown to be injurious to bermudagrass during “greenup”. The other products listed belong to a relatively new class of herbicides known as sulfonylureas (SU’s). This class of herbicides inhibits the acetolactate synthase (ALS) enzyme in plants. For highly susceptible species of plants, such as perennial ryegrass, the metabolism of the compound is much slower than less susceptible species, such as bermudagrass. SU’s are currently the most studied and used transition herbicides on the market. Over 40 field research trials have been conducted by researchers at Virginia Tech on various golf courses around the Commonwealth to evaluate transition herbicides. The following recommendations area based on those studies.

Timing and Rate of herbicide application.

Many will tell you to treat at “50% greenup.” Have you ever asked yourself what that really means? In fact, it’s a useless term. If you overseed with perennial ryegrass, isn’t all of the turf green? When you dig down in the perennial ryegrass you may find numerous green bermudagrass shoots but it is almost impossible to determine percentage bermudagrass greenup in an overseeded situation. Treatment timing is typically dictated by sports schedules and events rather than supposed bermudagrass “greenup.” If you’re an athletic field manager, you might treat much earlier than a golf course superintendent because you can live with terrible turfgrass quality while the bermudagrass grows to cover bare areas. Keep these points in mind when considering application timing: 1) bermudagrass and ryegrass must be actively growing when you treat, 2) When nonoverseeded bermudagrass is at 50% greenup, you probably have about 10 to 25% green bermudagrass cover under your perennial ryegrass fairway canopy, 3) to limit the duration of reduced turfgrass aesthetics, treat when bermudagrass is actively competing with ryegrass and breaching the ryegrass canopy (usually late May to late June), 4) promote bermudagrass growth by aggressive fertility and vertical cutting during or preferably before the transition treatment, and 5) bermudagrass needs about 100 days of active growth to fully recover from injury sustained by overseeding (e.g., if you “spray out” on June 1, then you should wait until September 10 to overseed). In general, higher herbicide rates are needed on thin a thin bermudagrass base to compensate for the lack of bermudagrass competition with ryegrass. However, keep treatment rates within the label specified range or injury could result. Each herbicide has its’ own strengths and weaknesses. For example, Revolver is extremely safe to bermudagrass and effective against clumpy Poa and perennial ryegrass but does not control most broadleaf weeds. Monument controls most Poa and perennial ryegrass as well as sedges and many broadleaf weeds. More information about these herbicides is available from Dr. Askew’s website at Virginia Tech (www.turfweeds.net).

Summary.

Applying herbicides for aid in spring transition from a ryegrass cover to a pure stand of bermudagrass is extremely useful, but needs to be implemented with caution. Below is a list of recommendations to consider before transitioning with an herbicide.

Ø Apply treatments to actively growing turf but beware of poor aesthetics that result from an underdeveloped bermudagrass base.

Ø Effective herbicide rates increase as bermudagrass density decreases or ryegrass age increases.

Ø Herbicides are more effective under conditions of adequate soil moisture and warm temperatures.

Ø Be prepared to deal with yellowing of the ryegrass and bare spots until bermudagrass fully fills in (usually 2 to 4 weeks).

Ø Fertilize based on a bermudagrass “grow in” regime to reduce the period of poor aesthetics due to bare spots.

Ø Allow for 100 days of bermudagrass growth.

Ø Beware of lateral movement potential and irrigate treated areas that border sensitive turfgrass before tracking or natural rainfall occurs.

When using these products or other pesticides, always read and follow all labeling instructions.