Shades of green
Among cool-season turfgrasses, creeping bentgrass possesses only a medium amount of shade tolerance. On shaded tees, fairways and greens, this factor, coupled with traffic, divoting and increased disease pressure, often results in poor bentgrass performance. Plants grown in shade receive less light for photosynthesis and thereby store less energy (carbohydrates). This fact, coupled with an undesirable increase in leaf elongation and succulence in the shade (due to increased gibberellic acid (GA) production), imply that the plant has less stored energy, in terms of roots and stolons, to persist and recover from use. A common observation is that shaded bentgrass plants are top-heavy: they put all of their energy into maintaining a shoot system, resulting in a very shallow (less than 2 inches deep) root system. Due to decreased evapotranspiration (ET) in the shade, shallow roots, during adequate rainfall periods, may not pose a problem. However, if drought occurs, shallow roots, coupled with tree-root competition, can spell disaster.
Forestalling the decline of shaded creeping bentgrass will take an integrated approach. The list below details some standard and non-standard cultural and chemical approaches you can use to keep bentgrass healthy.
- Sunlight and air movement
Selectively remove trees, prune limbs and thin bushes to increase sunlight and reduce leaf wetness periods. Focus thinning efforts in a way that increases morning sun for faster removal of dew and frost.
- Tree roots
Trench or vibratory plow to a 3-foot depth to sever tree roots around the perimeter of affected greens, tees or fairways. Repeat every five years.
Moister soil and a shallow-rooted plant with softer, elongated leaves call for frequent mowing with lightweight equipment and sharp blades. Reduce sharp turns and cleanup lap frequency and raise mowing height by an eighth to a quarter of an inch to increase photosynthetic leaf area.
- Nitrogen (N)
Shallow roots imply less of an ability to efficiently use higher amounts (more than 0.5 lbs N/M) of soluble N at one time. Apply lower amounts of soluble N more frequently or utilize slow-release N sources.
- Phosphorus (P)
Many samples for soil tests are taken to a depth of 4 to 6 inches. These results over-estimate available P on a 2-inch rooted turf. Sample only to effective rooting depth and apply recommended P accordingly.
- Potassium (K)
Adequate K builds strong cell walls. Keep K levels high in shade to encourage tougher leaf blades.
Shade-induced leaf-blade elongation dilutes leaf chlorophyll concentration. Spoon-feed with iron to increase leaf color (chlorophyll levels) and improve light-use efficiency.
- Irrigation and Drainage
Irrigate infrequently to moisten, not saturate, soil to a depth just deeper than the effective root zone. Install drainage to reduce the potential of standing-water events.
- Traffic and Divots
Move tee markers and cups daily and direct cart traffic away from shaded fairway areas. Implement a golfer and maintenance crew program of filling divots with sand.
Implement a 7- to 14-day preventive fungicide rotation program, especially for leaf spot, dollar spot and brown patch diseases.
Poa annua and winter annual broadleaves may be present. Because there are no selective herbicides for Poa, discourage Poa infestation by utilizing a PGR program and by aerating, overseeding with bentgrass and topdressing in late summer. Control broadleaves with bentgrass formulations of auxin-type herbicides.
- Aeration and Topdressing
Aerify less frequently than you do for turf in full sun to relieve compaction or prepare area for overseeding. Thatch build-up should not be an issue. Lightly sand topdress as needed during cooler periods to retain smoothness and promote overseeding success.
- Insects and Earthworms
White grub populations are normally higher in moist soils, so preventive applications of halofenozide (Mach 2) or imidacloprid (Merit) are probably justified. These two insecticides will not injure earthworms. To kill earthworms you can use Carbaryl (Sevin), ethoprop (Mocap) and fonofos (Crusade), which are highly toxic to earthworms.
- Plant Growth Regulators (PGRs)
Multiple research projects have indicated that anti-GA PGRs sustain bentgrass density in the shade. Apply flurprimidol (Cutless) at 0.25-0.5 lb/A or paclobutrazol (Trimmit) at 16-32 oz./A every 2 to 4 weeks during spring and fall for sustaining density and Poa suppression. Apply trinexapac-ethyl (Primo Maxx) at 6-12 oz./A every 2 to 4 weeks during summer for sustaining density.
My research at Virginia Tech has indicated very minor to no benefits of bi-weekly applications of seaweed, humic-acid or amino-acid based biostimulant materials on bentgrass shade tolerance.
- Algae and Moss
You should be able to minimize problems with algae and moss by paying proper attention to the suggested cultural and chemical approaches above. However, extended periods of wet, cloudy weather may override even the best efforts. In these cases, careful use of products containing copper hydroxide, ferrous sulfate or potassium salts of fatty acids may serve to keep algae and moss in check.
Integration of these approaches into an annual shaded bentgrass management plan will not guarantee success. Further, one or two major mechanical damage events from poor mowing practices or lack of cart control on shade-weakened bentgrass can negate months of successful cultural and chemical maintenance. If this occurs, your only choice may be to close the area and start over by sodding or re-seeding. Finally, remember that with any turfgrass problem long-term success will only be realized by identifying the primary factors related to decline and minimizing their deleterious effects; otherwise, you'll have to convince golfers that they like to play off dirt!
Erik H. Ervin, Ph.D., is assistant professor of turfgrass physiology at Virginia Polytechnic Institute and State University (Blacksburg, Va.).
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