March 23, 2023
To mow or not to mow in May
By Dr. Sara Stricker, Communications and Outreach Coordinator, Guelph Turfgrass Institute
& Dr. Eric Lyons, Director, Guelph Turfgrass Institute
The “No Mow May” campaign encourages homeowners to not mow their lawns for the month of May. This movement was started in the United Kingdom to support insect pollinator populations. The theory is that pollinators need sources of food in the spring and lawns could provide this food through flowering weeds, thereby increasing the abundance of pollinators. This movement has made its way into Ontario and is being promoted by conservation authorities. The experts at the Guelph Turfgrass Institute (GTI) have been asked to answer the question of whether the practice of No Mow May makes sense for landscapes in Ontario.
One research study, which has since been retracted due to errors, stated that No-Mow spaces exhibited more bees and more bee diversity than frequently mowed greenspaces1. That study is one of the few research papers supporting the concept of No Mow May and yet has serious shortcomings regarding methodology and potential conclusions. By design, the experiment was flawed to begin with. The researchers had compared No-Mow home lawns to municipal parks, which are vastly different ecosystems. Home lawns typically have more diversity to begin with because of their proximity to flower beds or flowering trees, compared to the centre of a field in a park. A better comparison would have been to compare lawns participating in No Mow May to lawns that were not. Stay tuned for results of such a study currently underway at GTI.
One of the first issues with the No Mow May movement is that Ontario and most of Canada have very different weather patterns and different species growing in lawns than in the United Kingdom. Many of the pollinator-friendly flowers studied by the originator of this movement are either non-native (such as knapweed and bird’s-foot trefoil) or are rarely found in Ontario lawns (such as oxeye daisy and orchids)2. The most common weed found in our lawns is the dandelion, and these yellow flowers are an incomplete source of nutrition for bees3. When it comes to feeding pollinators, diversity is key4. In fact, the most plentiful early-season source of pollen in Ontario comes from maple, elm, oak, crabapple, choke cherry and willow trees5,6,7.
The second, and perhaps more serious issue, is that No Mow May can be harmful to the home lawn. By allowing the lawn to exceed the height recommended before cutting (4.5-inches) for a cut height of three inches, you may encourage the presence of undesirable weed species like thistles, prostrate knotweed, and wild violet. Once these weeds establish in a lawn, the only answer is meticulous and continuous manual removal — possibly paired with Allowable List pesticides. Eventually, this can lead to homeowners giving up on their lawns entirely, potentially replacing them with rocks or hardscapes. Replacing greenspaces with hardscapes increases the risk of flooding, prevents rainwater from replenishing underground aquifers, and increases the ambient heat in the area.
Best practices recommend to cut no more than one-third of the grass foliage at any time. You can imagine how catastrophic it would be to suddenly lose a huge source of food and energy (i.e., photosynthesizing surface). Depending on weather conditions, turf might be actively growing in April and May, and reach heights over six inches tall. A typical walk-behind mower has a maximum cutting height of around three inches, so a mowing event would remove half the plant. This would be detrimental to the health of the turf, making the lawn more susceptible to insect, weed and disease pressures. Letting the grass grow tall will also make it more difficult to mow with a standard mower, which can lead to a buildup of dead material (thatch), and increase the risk of “scalping” the turf. Scalping is the removal of an excessive quantity of leaf tissue at any one mowing, resulting in a brown appearance due to exposed stems, stolons, crowns, dead leaves, or bare soil. It is also recommended to evenly disperse grass clippings across your lawn after mowing since clumps of clippings can smother and kill the grass below.
Excessive grass clippings, especially when clumped together, can smother the grass.
Rodents: Mice and rats love an unmowed lawn. Not only will these pests sneak into your home to steal food, but they can cause excessive damage to buildings by gnawing wood, plastic, and electrical wires. Plus, they can carry diseases and fleas.
Biting insects: Fleas, ticks and mosquitoes are commonly found in moist and shady areas. Tall grass can provide habitats for these unwanted pests which can carry and transmit West Nile virus, Lyme disease, and heartworms to humans and pets. This is one of the important reasons municipalities have established bylaws stating mowing heights within city/town limits.
Mental health: Research suggests a link between positive mental health and greenspaces. This includes looking at greenery, access to outdoor recreation activities, and breathing the cool, clean air provided through photosynthesis.
Converting parts of a yard to a restored prairie or savannah is another wonderful way to support biodiversity in your backyard. The function of these naturalized areas is vastly improved if left undisturbed year-round, which supports overwintering and breeding of insects. Many native bee species prefer to nest in rotting wood, hollow stems of flowers (like sunflowers), and undisturbed soil. With that in mind, long-grass areas can also be a breeding ground for unwanted insects such as mosquitos, ground-nesting bees and ticks. Keep an eye out for these pests, prevent kids and pets from entering naturalized areas, and try to locate them farther from your house.
Planting and maintaining pollinator-friendly plants is a good practice but that can be achieved while maintaining the usefulness and sustainability of the lawn. Traditional turfgrass lawns exist because of their utility. They serve as a place for kids to play and for the whole family to gather for activities. Lawns do this while still being a living plant that contributes ecological benefits to the urban environment.
2. Plantlife International. 2023. Available online: https://www.plantlife.org.uk/uk
3. Loper, G.M. and A.C. Cohen. 1987. Amino acid content of dandelion pollen, a honey bee nutritional evaluation. Journal of Economic Entomology 80 (1):14–17.
4. Ostaff, D.P. et al. 2015. Willows as pollen and nectar sources for sustaining fruit and berry pollinating insects. Canadian Journal of Plant Science 95(3): 505–516.
5. Affek, A.N. et. al. 2021. Pollinator potential of riparian hardwood forests – a multifaceted field-based assessment in the Vistula Valley, Poland. Forests 12: 907.
6. Batra, S.W. 1985. Red maple an important early spring food resource for honey bees and other insects. Journal of the Kansas Entomological Society 58(1): 169–172.
7. Beard, J. B. 1973. Turfgrass: Science and Culture. Prentice-Hall International, London.
8. Donkersley, P. et al. 2017. Nutritional composition of honey bee food stores vary with floral composition. Oecologia 185: 749–761.
9. Credit Valley Conservation. 2017. Native Plants for Pollinators. Available Online via https://cvc.ca/
10. Hicks, D.M. et al. 2016.Food for pollinators: Quantifying nectar and pollen reserves of urban flower meadows. PLoS One 11(6): e0158117.
& Dr. Eric Lyons, Director, Guelph Turfgrass Institute
The “No Mow May” campaign encourages homeowners to not mow their lawns for the month of May. This movement was started in the United Kingdom to support insect pollinator populations. The theory is that pollinators need sources of food in the spring and lawns could provide this food through flowering weeds, thereby increasing the abundance of pollinators. This movement has made its way into Ontario and is being promoted by conservation authorities. The experts at the Guelph Turfgrass Institute (GTI) have been asked to answer the question of whether the practice of No Mow May makes sense for landscapes in Ontario.
One research study, which has since been retracted due to errors, stated that No-Mow spaces exhibited more bees and more bee diversity than frequently mowed greenspaces1. That study is one of the few research papers supporting the concept of No Mow May and yet has serious shortcomings regarding methodology and potential conclusions. By design, the experiment was flawed to begin with. The researchers had compared No-Mow home lawns to municipal parks, which are vastly different ecosystems. Home lawns typically have more diversity to begin with because of their proximity to flower beds or flowering trees, compared to the centre of a field in a park. A better comparison would have been to compare lawns participating in No Mow May to lawns that were not. Stay tuned for results of such a study currently underway at GTI.
One of the first issues with the No Mow May movement is that Ontario and most of Canada have very different weather patterns and different species growing in lawns than in the United Kingdom. Many of the pollinator-friendly flowers studied by the originator of this movement are either non-native (such as knapweed and bird’s-foot trefoil) or are rarely found in Ontario lawns (such as oxeye daisy and orchids)2. The most common weed found in our lawns is the dandelion, and these yellow flowers are an incomplete source of nutrition for bees3. When it comes to feeding pollinators, diversity is key4. In fact, the most plentiful early-season source of pollen in Ontario comes from maple, elm, oak, crabapple, choke cherry and willow trees5,6,7.
The second, and perhaps more serious issue, is that No Mow May can be harmful to the home lawn. By allowing the lawn to exceed the height recommended before cutting (4.5-inches) for a cut height of three inches, you may encourage the presence of undesirable weed species like thistles, prostrate knotweed, and wild violet. Once these weeds establish in a lawn, the only answer is meticulous and continuous manual removal — possibly paired with Allowable List pesticides. Eventually, this can lead to homeowners giving up on their lawns entirely, potentially replacing them with rocks or hardscapes. Replacing greenspaces with hardscapes increases the risk of flooding, prevents rainwater from replenishing underground aquifers, and increases the ambient heat in the area.
Best practices recommend to cut no more than one-third of the grass foliage at any time. You can imagine how catastrophic it would be to suddenly lose a huge source of food and energy (i.e., photosynthesizing surface). Depending on weather conditions, turf might be actively growing in April and May, and reach heights over six inches tall. A typical walk-behind mower has a maximum cutting height of around three inches, so a mowing event would remove half the plant. This would be detrimental to the health of the turf, making the lawn more susceptible to insect, weed and disease pressures. Letting the grass grow tall will also make it more difficult to mow with a standard mower, which can lead to a buildup of dead material (thatch), and increase the risk of “scalping” the turf. Scalping is the removal of an excessive quantity of leaf tissue at any one mowing, resulting in a brown appearance due to exposed stems, stolons, crowns, dead leaves, or bare soil. It is also recommended to evenly disperse grass clippings across your lawn after mowing since clumps of clippings can smother and kill the grass below.
Excessive grass clippings, especially when clumped together, can smother the grass.
Lawns and your health
The lawn can be traced back to medieval Europe, where grassy areas surrounded castles and villages8. These greenspaces functioned as a shared area where sheep or cattle could graze, or as a defensive tactic against enemy ambush. Later, the close-cropped grassy surface allowed for the invention of sports such as golf, cricket, soccer, and lawn bowling. However, there are additional and unforeseen benefits of surrounding your home with a mowed lawn.Rodents: Mice and rats love an unmowed lawn. Not only will these pests sneak into your home to steal food, but they can cause excessive damage to buildings by gnawing wood, plastic, and electrical wires. Plus, they can carry diseases and fleas.
Biting insects: Fleas, ticks and mosquitoes are commonly found in moist and shady areas. Tall grass can provide habitats for these unwanted pests which can carry and transmit West Nile virus, Lyme disease, and heartworms to humans and pets. This is one of the important reasons municipalities have established bylaws stating mowing heights within city/town limits.
Mental health: Research suggests a link between positive mental health and greenspaces. This includes looking at greenery, access to outdoor recreation activities, and breathing the cool, clean air provided through photosynthesis.
Ways to support pollinators
We recommend homeowners add pollinator-friendly flowers, trees and shrubs to their yards. Easy to grow native flowers include: asters, beardtongue, coneflower, and goldenrod9. Other highly nutritious pollen sources available in the spring include white clover and creeping buttercup, which can be easily seeded into a home lawn and can establish a harmonious relationship even when mowed10. Planting crocus bulbs into a lawn can provide pollen in March and April, and the flowering period is typically finished before the grass needs its first trim. Admittedly, clover and crocus are not native to North America, but are commonly found across Ontario.Converting parts of a yard to a restored prairie or savannah is another wonderful way to support biodiversity in your backyard. The function of these naturalized areas is vastly improved if left undisturbed year-round, which supports overwintering and breeding of insects. Many native bee species prefer to nest in rotting wood, hollow stems of flowers (like sunflowers), and undisturbed soil. With that in mind, long-grass areas can also be a breeding ground for unwanted insects such as mosquitos, ground-nesting bees and ticks. Keep an eye out for these pests, prevent kids and pets from entering naturalized areas, and try to locate them farther from your house.
Maximizing turf health
We recommend mowing turf when it reaches approximately three inches high. Mowing the grass to two inches then stays within the “one-third rule” and keeps the turf actively growing. Since grasses evolved to be grazed upon by herbivores, they will actually respond positively to mowing practices. Spring is also a good time to remove any debris or leaves that may have been left from the fall, although we do not recommend aggressive raking in the spring unless there appears to be a need to remove matted dead grass.Planting and maintaining pollinator-friendly plants is a good practice but that can be achieved while maintaining the usefulness and sustainability of the lawn. Traditional turfgrass lawns exist because of their utility. They serve as a place for kids to play and for the whole family to gather for activities. Lawns do this while still being a living plant that contributes ecological benefits to the urban environment.
Sources used for this publication include:
1. Del Toro, I. and R.R. Ribbons. 2020. Now Mow May lawns have higher pollinator richness and abundance: An engaged community provides floral resources for pollinators. PeerJ 8: e10021 (RETRACTED NOV 18, 2022).2. Plantlife International. 2023. Available online: https://www.plantlife.org.uk/uk
3. Loper, G.M. and A.C. Cohen. 1987. Amino acid content of dandelion pollen, a honey bee nutritional evaluation. Journal of Economic Entomology 80 (1):14–17.
4. Ostaff, D.P. et al. 2015. Willows as pollen and nectar sources for sustaining fruit and berry pollinating insects. Canadian Journal of Plant Science 95(3): 505–516.
5. Affek, A.N. et. al. 2021. Pollinator potential of riparian hardwood forests – a multifaceted field-based assessment in the Vistula Valley, Poland. Forests 12: 907.
6. Batra, S.W. 1985. Red maple an important early spring food resource for honey bees and other insects. Journal of the Kansas Entomological Society 58(1): 169–172.
7. Beard, J. B. 1973. Turfgrass: Science and Culture. Prentice-Hall International, London.
8. Donkersley, P. et al. 2017. Nutritional composition of honey bee food stores vary with floral composition. Oecologia 185: 749–761.
9. Credit Valley Conservation. 2017. Native Plants for Pollinators. Available Online via https://cvc.ca/
10. Hicks, D.M. et al. 2016.Food for pollinators: Quantifying nectar and pollen reserves of urban flower meadows. PLoS One 11(6): e0158117.