Intraseeding putting greens--does it work?
Creeping bentgrass (Agrostis palustris) is the predominant turfgrass used on putting greens, providing a uniform, high-quality putting surface. However, the demand for faster green speeds and higher-quality, more stress-tolerant turf has led to the development of many new cultivars.
Superintendents interested in converting their greens to one of these new cultivars face a dilemma. How do you successfully introduce a new variety into your greens without disrupting play? Complete renovation, in which the existing turf is killed or stripped, is perhaps the most effective way to ensure the establishment of a new cultivar. However, budgetary constraints and the lost revenue resulting from closing greens may preclude this option.
Alternatively, many superintendents attempt to convert their established putting greens to new cultivars via intraseeding. Intraseeding is the introduction of seed into an established population of the same species without killing the existing stand. This process entails cultural and chemical strategies to weaken the existing turf and lessen its competitive advantage over the seedlings of the new variety. Ideally, this should promote a population shift to the new variety with only minimal disruption to the playing surface.
Reports of successful intraseeding have been based on visual observations of post-treatment performance of greens. However, because it is almost impossible to visually distinguish between creeping bentgrass cultivars intermixed in a stand, these observations are questionable and the value of intraseeding remains in doubt.
Turfgrass ecology and competition To understand how intraseeding works (or not), it is important to first look at a golf green not as a whole, but as a population of individual plants that continually interact. Each species requires a certain range of resources and conditions to survive and reproduce. This defines its niche, which is dictated by the species' needs for resources such as light, nutrients, space, air and water. Other important factors (on golf greens) include temperature, soil pH and mowing height. Competition arises when individual plants with similar niches must share limited resources, which often leads to reduced survival, growth and reproduction.
When two individuals (or species) require exactly the same niche, the one that is better adapted to existing conditions will ultimately win out. However, as long as there is at least some difference in the resources and conditions they require, then they may continue to coexist, perhaps indefinitely, without any clear "winner."
Such is the case in mixed stands of annual bluegrass and creeping bentgrass. These two species can coexist on a green because their requirements, though somewhat similar, are different enough to allow both to survive. However, if resources and conditions change, a shift in the turfgrass community can occur and the species that is better adapted to the new conditions will gain an advantage.
For example, increasing the proportion of nitrate nitrogen (NO3-) to ammonium (NH4+) increases growth and tillering of annual bluegrass, but has little effect on the creeping bentgrass. Thus, by manipulating the nitrogen source, you can enhance the competitive ability of annual bluegrass over that of creeping bentgrass.
Certain weeds and grasses that occur on bentgrass fairways but not typically on creeping bentgrass putting greens also illustrate this point. The difference is mowing heights. Grasses and weeds that cannot tolerate lower heights succeed with higher mowing heights.
When you understand that competition between plants with highly similar requirements inevitably leads to winners and losers, a couple of potential problems with intraseeding become apparent.
Competition between individuals of the same species (creeping bentgrass) is intense due to their highly similar requirements. This is the situation that exists in a stand composed of more than one cultivar of creeping bentgrass, which, of course, is what you get when you intraseed a golf green.
The new variety is not necessarily going to be more competitive. In fact, the cultivar you've just seeded is at a disadvantage simply because the plants are starting out as tiny seedlings competing with the mature, multi-tillered plants already present.
Research backs this up. Studies have shown that grass seedlings that were sown into established swards experienced poor survival, largely due to root competition (rather than competition for light, as you might initially suppose). Further, these studies showed that early seedling survival was enhanced with larger "gap" sizes that reduced root and shoot competition. Unfortunately, creating these large gaps can result in significant disruption of the putting surface, defeating the purpose of intraseeding. In addition, such gaps may lead to genetic patchiness in the stand.
We decided to conduct further studies to see if alternative techniques could reduce the competitive advantage of the established turf to aid the newly intraseeded cultivars.
Researching intraseeding At the Ohio Turfgrass Foundation Research and Education Facility, we established two 20 x 40 foot USGA-style greens with Penncross creeping bentgrass maintained at 5/32 inch. Each green was divided into five treatment plots, arranged in a randomized complete block design. The treatments, in conjunction with intraseeding, included:
* Glyphosate (Roundup) applied at 3 ounces per 1,000 square feet 7 days prior to seeding. * Trinexapac-ethyl (Primo) applied at 0.33 ounce per 1,000 square feet at least 24 hours prior to seeding to allow maximum absorption and translocation within the plants. * Coring with 0.5-inch hollow tines at a 3-inch depth, with 2-inch spacing within rows and 2.5 inches between rows. After removing the cores, we rolled the plots to smooth the surface. * Scalping with repeated (20 to 30 times) mowing at 1/16 inch, leaving little to no verdure and exposing crowns and stems. * Control plot received only the seed.
On the day of these treatments, we double-cut the greens at 1/8 inch with a walking greens mower. All treatments, excluding the untreated control, were then vertically mowed in two directions with 1/8-inch-thick blades on 0.75-inch spacing set to cut 0.25-inch-deep grooves into the surface. We removed the loose debris and topdressed the plots (again, excluding the control) with a moderate layer of sand, brooming it to work it into the canopy.
These treatments are similar to those that many superintendents use to overseed bermudagrass greens in the fall. The idea behind these practices is to weaken the existing turf while promoting good seed-to-soil contact. The glyphosate treatment served as a check plot for monitoring the success of seedling establishment without competition.
We intraseeded the plots in September 1998, April 1999 and September 1999. We applied Penn G-2 creeping bentgrass seed in two directions at a rate of 0.25 pound per 1,000 square feet. Light brooming helped work the seed into the verticut grooves and aerification holes to provide good seed-to-soil contact.
Post-treatment management of the greens focused on promoting maximum seed germination and seedling establishment with light, frequent irrigation and sequential applications of a starter fertilizer. We temporarily raised mowing heights to 5/32 inch and used solid rollers to minimize damage to seedlings. After 4 weeks, we slowly lowered heights back down to 1/8 inch to reduce competition for light.
We sampled the plots at the end of May 1999 and in March 2000 to determine how well the seedlings had become established. To ensure accurate identification of grass cultivars, we used RAPD (Random Amplified Polymorphic DNA) markers, a tool for "fingerprinting" genetic types, to distinguish between members of the two creeping bentgrass cultivars.
Nearly all creeping bentgrass cultivars are synthetic, meaning that the seed results from the cross-pollination of several superior parent types. Therefore, no two individuals of a synthetic cultivar are genetically the same. As a result, when employing RAPDs, it is necessary to use genetic bands, or markers, that consistently occur in one cultivar but not the other.
Results After several months of screening, we found a distinct genetic marker that occurred consistently in Penn G-2 but not in Penncross. This allowed us to process DNA samples from each treatment to determine how much of each cultivar was present.
To date, our results show little or no evidence of the Penn G-2 cultivar in any of the plots, other than those treated with glyphosate. These results suggest that mechanical surface disruption and applications of plant growth regulators have limited impact when it comes to intraseeding establishment.
You might imagine that with almost complete defoliation of plants from scalping, seedling establishment would be successful. However, the reason for failure may be that the treatments did not eliminate root competition for nutrients and soil moisture. Then, as the weakened existing grass plants recover and the canopy closes, the seedlings lose out, ultimately resulting in little or no change in the population.
In fact, we initially observed seedlings in the verticut grooves and coring holes. However, newly germinated seedlings do not necessarily compete with neighboring plants. Instead, they may live off stored reserves remaining in the seed. True competition does not begin to play a role until these seedlings use up their reserves and must begin producing their own resources through photosynthesis in their leaves. The results of our study show that once this happens, the juvenile G-2 plants do not compete successfully with the established Penncross.
By contrast, our results show that chemical renovation can be highly successful at ensuring the establishment of the new cultivar. The glyphosate applications resulted in an almost complete conversion to G-2. In this case, the herbicide eliminated shoot and root competition, providing conditions conducive for seedling establishment.
Superintendents typically think of complete conversions as gradual, long-term processes, with successive treatments adding more of the new cultivar over time. However, our study shows that the number of surviving seedlings after each treatment is trivial. If we extrapolate from this, repeated treatments will probably yield little additional success.
Intraseeding has shown to be of little success in converting putting greens to new cultivars. Severe disruption or complete kill of the stand, followed by significant limitations of play, may be necessary for the establishment of these cultivars. Superintendents thinking about intraseeding should consider whether their effort and money would be more effectively spent on other tactics that could improve their putting greens.
This is not to discourage conversions to newer varieties. Greens consisting of older cultivars that respond poorly to improved cultural practices or that have become difficult to maintain may be good candidates for conversion. But superintendents should recognize that a complete renovation may be necessary to ensure successful conversion. Even though complete renovation projects may incur higher costs or lost revenue, you can be assured you're getting what you're paying for.
Dan Kendrick is a graduate research associate and Dr. Karl Danneberger is professor of turfgrass ecology, both at The Ohio State University (Columbus, Ohio). Dr. Patricia Sweeney, who is no longer with The Ohio State University, also participated in this research.
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