A common approach to the use of autonomous mowers involves large mowers minus the human controlling it — as is the case with this Graze (www.grazemowing.com/) large autonomous mower. Photos by J. Scott McElroy
Mowing is the most important turfgrass management practice. Without mowing, turfgrass is not really turf; it is just a pasture or some other unmanaged grass landscape. Mowing is what defines turf. This is basic Turfgrass Management 101.
To those who mow the grass and have studied how mowing affects turfgrass plants and swards, it is not just cutting the grass, but how we cut the grass. The three main components of mowing are height, frequency and mower type. These three factors are linked,
and manipulating one often means one must manipulate another. Lowering the mowing height likely means one has to increase mowing frequency per week and at some point means that a reel mower may be needed to increase the precision of cut at the lower
heights. Infrequent mowing, at any height or with any mower, can result in a decrease in turfgrass quality by removing the newest, uppermost leaves and exposing the older, lower, senescing leaves.
Mowing not only keeps a consistent height; it is also beneficial to the turf. Mowing disrupts apical dominance resulting in lateral growth and denser turfgrass. Mowing, along with traffic or cultivation on turfgrass, is vital to sustained quality of the
turfgrass. Mowing and traffic/cultivation stimulates new growth and removes less vigorous old growth. In order to maintain a healthy turfgrass stand, one must use the turfgrass stand. In essence, if you don’t use it, you lose it.
When it comes to mowing, there is one rule that is taught above all. It is one rule that rules them all: the one-third rule.
A fleet of lightweight autonomous mowers (or LAMs), like this one from Husqvarna (www.husqvarna.com/us/robotic-lawn-mowers/) has the potential to transform commercial mowing.
The one-third rule
One of the most basic principles of turfgrass management is the one-third rule, which states that at any mowing event, no more than one-third of the above-ground leaf material should be removed. If a turf stand is 3 inches in height, one would not want
to remove more than 1 inch of leaf material. But why?
There are two basic reasons why the one-third rule is a good rule of thumb for when to mow and at what height to mow turf.
First is stress. The removal of photosynthetic leaf material severely stresses the turfgrass stand. The ability of the plant to photosynthesize is compromised when excessive leaf material is removed, resulting in reallocation of sugars from roots or perennial
structures to induce new growth. Root growth can be compromised, resulting in less stress tolerance.
The second reason for the one-third rule is aesthetic value. As turfgrass grows vertically, lower leaves that are shaded begin to senesce, losing green color and moving mobile nutrients out of older leaves to new leaves. By excessively removing newer,
upper leaves, the senescent lower leaves are exposed, making the overall turfgrass stand appear light green, brown and thin — which, as stated, causes stress because of the loss of photosynthetic leaf material. Excess leaf removal leaves voids
in the turfgrass stand, allowing weeds to grow and compete with the recovering turfgrass.
The one-third rule may be the standard for the maximum amount of leaf area that can be removed, but it is certainly not the optimum amount of leaf to remove. The optimum amount of leaf material to remove at each mowing is as little as possible. The primary
way to remove less leaf material at each mowing is to increase mowing frequency. By mowing more frequently, less leaf material is removed at each cutting.
Figure 1. Underside of LAM showing small blades. In this version, blades are protected by a disc below the blades. The small blades cut as the rotating disc spins.
The one-third rule is not perfect
Despite being one of the most basic tenants of turfgrass management, the one-third rule is not perfect. For example, higher mowing frequency at lower heights in turf increases traffic, labor and fuel costs and requires more technical equipment. More mowing
with heavy equipment increases traffic on turf, which can begin to create negative wear on the turfgrass, increase compaction and decrease soil pore space and water infiltration. Increased mowing means increased labor for mowers and increased fuel
costs. Mowers require greater attention to maintain the precision of cut, requiring even more labor costs.
The solution for those who do not have the time, labor or money to increase mowing frequency is to increase height, thus decreasing frequency.
Even if we raise heights to the extremes, this can cause problems. For example, tall fescue home lawns are often recommended to be mowed at 3 to 4 inches (8 to 10 centimeters). At this height, lodging can occur, and tracks are made with heavy equipment.
Plus, consumers may not think that a 4-inch height is aesthetically pleasing. But such high mowing heights are only for residential and nonresidential lawns.
Golf courses and athletic fields have no choice but to maintain low heights at high frequency.
Figure 2. Because autonomous mower blades (from top, installed, flat and edge-on) are thinner and cut less material due to increased mowing time, they stay sharper longer and cut more precisely. Blades can also be changed in just minutes if needed.
Autonomous mowing is the future
It is our opinion that the way mowing is used in turfgrass management will completely change with autonomous mowers. We believe that people are discussing and looking at autonomous mowing simply as an extension of current mowing practices — a practice
that really just removes a human controlling the equipment. Autonomous mowers will be potentially more impactful than this. Autonomous mowers have the potential to completely revolutionize the approach to mowing and will change the aesthetic quality
of the turf, the impact on pests, and potentially the way we fertilize and use secondary cultivation practices such as aerification and verticutting. The way to make a transformative change is with the use of lightweight autonomous mowers, or LAMs.
Companies are thinking about adapting current technology — large rotary or reel mowers — to autonomous technology. For example, there are large-deck rotary mowers being developed as autonomous mowers. Indeed, many turfgrass managers think
that they will be purchasing equipment such as this that basically does the same thing that their current mowers do, but in the future they will be autonomous. Our question is, why think about adapting current mower technology to autonomous mowers?
Why not rethink what a mower should be as autonomous technology is developing?
Simply thinking of autonomous mowing as just removing the driver may be missing an opportunity to change turfgrass management practices. Traditional technology, retrofitted with autonomous equipment, likely still will have many of the same problems of
current technology. It will require the same type of mower maintenance to maintain cutting blades, and it will be heavy, causing compaction and wear.
Also, these types of mowers will be expensive, especially at the outset, and if the mower goes down, traditional mowers will need to be available to fill the gaps.
An alternative view is to transform turfgrass management to use LAMs, utilizing dozens of small mowers that will manage a golf course or other large-scale turfgrass complex, rather than single large mowers covering large areas.
Smaller mowers will cover smaller areas of 2-10 acres rather than having a few mowers doing the job. These swarms of mowers will be geolocated over a golf course with the ability to have mowers fill in for other mowers that need to be maintained. Managers
will monitor all the mowers on their phones or computer-based apps, getting real-time feedback regarding the activity, speed, battery life and location.
LAMs are small robotic mowers with cutting widths of approximately 6 to 30 inches (15 to 76 centimeters). They use rotating spinning discs with small (2-to-3-inch; 5-to-8-centimeter) blades that spin independently of the primary disc (Figure 1). LAMs
can be programmed to move randomly over a single area or multiple areas, moving back to the charging station when charge is low. LAMs can be programmed to mow anytime of the day or night and can mow areas every day. Currently, LAMs can mow between
approximately 0.4 and 4 inches (1 to 10 centimeters) in height, so putting greens are currently not possible commercially.
All of these unique aspects of LAMs will change not only the way we manage turfgrass, but also the turfgrass itself. Here are a few of the aspects of how management could change.
Constant cutting
LAMs can be programmed to mow every day at any hour, mowing every day, or potentially multiple times per day. Daily mowing can reduce the amount of clippings produced at each cutting. There will be less, if any, need for clipping collection. Smaller amounts
of clippings can more easily integrate into the canopy, improving nutrient recycling. Smaller amounts of clippings eliminate the need for clipping pick up, improving turfgrass quality. Because mowing occurs every day, the mowing height is always the
same and does not increase between mowing events.
Cleanness of cut
LAMs primarily cut turfgrass using a rotating spinning disc with attached blades. Three to four are attached per cutting disc. LAM blades require less sharpening and are easily replaced (Figure 2). Because they are smaller and cut less material on a daily
basis, they stay sharper longer, thus producing a clean leaf cut. Clean cuts improve the quality of the sward and potentially decrease the potential for fungal infections. Blade thickness (thinner blades) is the main reason for maintaining sharpness
even after prolonged use.
Turfgrass density and leaf architecture
One of the most intriguing aspects of the impact of autonomous mowers is the change it can cause in the turfgrass sward (Figure 3). Research suggests that constant mowing can decrease leaf width, increase leaf density and increase overall quality of tall
fescue (1). These changes increase the overall density of the sward and increase the quality. These changes likely will occur across different turfgrass species.
LAMs also improve the quality of perennial weed species and eliminate spontaneous infestation by other weed species. Species such as white clover (Trifolium repens) and bird’s-foot trefoil (Lotus corniculatus) develop denser, more turf-like stands
of higher quality under autonomous daily mowing compared to traditional rotary mowing (2) (Figure 4). Developing dense turf-like stands of select perennial herbaceous broadleaves could increase acceptance of plants that provide beneficial ecosystem-servicing,
such as pollinator attraction.
Figure 3. Solar charging stations can create independent power generation and eliminate the need to distribute power throughout a golf course. Trees, of course, will be the major obstacle to implementing solar power.
Energy usage
LAMs will use less energy than traditional mowers. LAMs are battery-powered, produce no local carbon dioxide and produce little to no noise. Research indicates that random-movement LAMs can mow up to seven times longer than traditional gas-powered rotary
mowers when managing tall fescue; however, the energy consumption for LAMs was one-third that of the rotary mower (1).
On-site solar charging can reduce the need to run power to locations all over a course (Figure 5). While this could increase the cost of installation, it would reduce power costs over time.
It is argued that battery-powered equipment and cars do not greatly reduce CO2 output compared to combustion engines since they primarily use power derived from coal-powered plants. First, this may not be true. Second, reduction in CO2 output can be reduced
by using small-scale on-site charging stations scattered over the course. Plus, energy consumption by LAMs is lower compared to gas-powered mowers.
Figure 4. White clover (Trifolium repens) sward development under weekly to biweekly rotary mowing (A) and daily mowing using a LAM (B).
Conclusions
In our final thoughts, we are going to hedge our bet a little. It is our opinion that small autonomous mowers could change turfgrass and the way it is managed. However, golf managers may choose large autonomous mowers over smaller mowers. But this we
are confident of: Autonomous mowers, regardless of the type, will change turfgrass management. While some changes, such as reduced numbers of mower operators and reduction of energy consumption with solar charging, are self-evident, others are not.
Early testing indicates that robotic mowers will change the use of mowers and will change the response of turf to mowing. Extended mowing time, increased mowing frequency and reduced soil compaction are just some of the new scenarios that open when
considering the introduction of this new technology, and new research needs to be completed before all aspects and potential benefits could be appropriately described and subsequently exploited for better turf management.
Figure 5. Daily mowing with LAMs physically changes the turfgrass improving turfgrass sward quality.
The research says
- Lightweight autonomous mowers (LAMs) will use less energy than traditional mowers. LAMs are battery-powered, produce no local carbon dioxide and produce little to no noise.
- Because LAM blades are smaller and cut less material on a daily basis, they stay sharper longer, thus producing a clean leaf cut.
- LAMs can cut turfgrass constantly, any time of the day, reducing clippings and maintaining a nearly constant turf height.
Literature cited
- Grossi N., M. Fontanelli, E. Garramone, A. Peruzzi, M. Raffaelli et al. 2016. Autonomous mower saves energy and improves quality of tall fescue lawn. HortTechnology 26(6):825-830 (https://doi.org/10.21273/HORTTECH03483-16).
- Pirchio M., M. Fontanelli, C. Frasconi, L. Martelloni, M. Raffaelli et al. 2018. Autonomous mower vs. rotary mower: Effects on turf quality and weed control in tall fescue lawn. Agronomy 8(2):15 (https://doi.org/10.3390/agronomy8020015).
J. Scott McElroy (mcelroy@auburn.edu) is a professor of weed science and turf in the Department of Crop, Soil and Environmental Sciences at Auburn University, Auburn, Ala. Simone Magni, Lisa Caturegli, Mino Sportelli, Giuliano Sciusco, Marco Fontanelli
and Marco Volterrani are researchers at CeRTES, the Center for Research on Turfgrass for Environment and Sports, in the Department of Agriculture, Food and Environment, University of Pisa, Italy.