The Right Mowing Height for a Durham Region Lawn

The usual explanation is that tall grass stops crabgrass seed from germinating because the seed needs light. That explanation is wrong, and the same Purdue paper is what proves it.

They tested it directly. Smooth crabgrass germination was 99 per cent or higher after exposure to blue light, red light, far red light, both red and far red, or complete darkness. Seed in the dark germinated just as happily as seed in the sun.

What the canopy actually does is starve the seedling after it comes up. In the deepest shade treatment, crabgrass plants produced no tillers at all, where plants in full sun produced an average of 2.1. The plant germinates, emerges into a dark, crowded, competitive canopy, and never gets big enough to matter or to set seed for next year.

The light numbers are worth having, because they explain why small height changes have large effects. Measured at the base of a ryegrass canopy, light reaching the soil was 5.4 to 7.5 per cent of full sunlight at 3.2 cm, 1.2 to 3.4 per cent at 5.7 cm, and 0.1 to 0.4 per cent at 8.3 cm. Between a short cut and a tall one the soil surface goes from dim to effectively dark, and it does it across about five centimetres of deck travel.

Purdue's own model turns that into a prediction: a canopy thin enough to let 50 per cent of light through was projected to carry 8 to 21 per cent crabgrass by summer, and one letting 75 per cent through, 21 to 46 per cent.

Here is what a company selling you a mowing service would normally leave out.

Cutting height does not suppress every weed. Some of them like it. A Penn State review reports a Maryland study in which tall fescue plots cut at 8.8 cm carried more white clover than plots cut at 3.2 and 5.5 cm. A Minnesota study found that mowing at 8.3 cm increased cover of birdsfoot trefoil and black medic compared with lower heights.

Those are broadleaf, low growing, nitrogen fixing plants. They are perfectly adapted to sitting under a taller canopy, and the shade argument that beats crabgrass does not touch them. The same Penn State paper also found that mowing frequency had no significant influence on weed cover at all, which is another tidy story that did not survive measurement.

What that means practically on a Durham lawn: if your complaint is crabgrass, cutting height is the strongest free tool you have. If your complaint is clover, cutting height is not going to fix it and anyone who tells you it will has not read the research. Density from an autumn overseeding is the answer to clover, and it is slower.

Never remove more than a third of the blade in a single cut. Ontario's turf manual says it twice. Purdue, Ohio State, Minnesota and Cornell all say it.

The University of Guelph publishes a worked example, and the numbers in it are the numbers we run: if your mowing height is 7.5 cm then you should mow the field before the turf reaches 11 cm tall. A Canadian university, in centimetres, using the exact height and the exact trigger.

Guelph also explains why the rule exists, and the explanation is the useful half of it. Grass grows from the base, so the youngest fully expanded leaves sit at the top of the plant. Let it get away from you and a single cut removes precisely those leaves, which are the ones photosynthesising hardest and contributing most to recovery, and leaves behind the old senescing tissue and the immature growth. That is why a late cut looks pale for a week. You did not just shorten the plant, you removed its best working parts.

Cornell publishes the intervals this produces: about every five days at 2 inches, every seven and a half days at 3 inches, and every ten days at 4 inches. Cutting short does not buy you a longer gap between visits. It buys you a shorter one.

That interval is why our lawn season runs 26 visits between May 1 and November 1 on a weekly schedule. At 7.5 cm the weekly interval and the one third rule line up almost exactly through the growing season, and the visit does not have to be renegotiated every May.

Ontario states the relationship in one line: rooting depth is proportional to mowing height, the longer the leaves, the deeper the roots. The province says the inverse on a second page: lower mowing produces a shallow root system, and shallow roots cannot take up enough water and nutrients, which is what makes a lawn fold in a dry August.

Purdue reduces it to seven words: the higher you mow a grass plant, the deeper the roots. Ohio State describes closely mown lawns as developing shallow root systems with less total mass. Health Canada, in metric, says grass cut at 6 to 8 cm will develop a deep, extensive root system and grow thicker.

That is the part that does not show up in July. It shows up in the third week of a dry August, on the two lawns on the same street where one is green and one is straw, and the decision that separated them was made in May.

A dull blade tears the leaf instead of cutting it. Purdue describes the result as a whitish cast visible across the lawn from a distance, and the torn tissue as an opening for pathogens. A 1983 study in Agronomy Journal ran Kentucky bluegrass under repeated mowing with sharp and dull rotary blades and found turf quality, shoot density and verdure all reduced under the dull treatment.

How often to sharpen is where the sources scatter. Purdue's bulletin says four to six times a year. Purdue's own turf blog says homeowners should manage at least twice. Ontario's manual just says always mow with a sharp blade. Cornell gives the only figure tied to something you can count: sharpen after ten hours of mowing. That is the one we use, because hours are measurable and a calendar is not.

And a check you can run yourself in two minutes, from Michigan State: roll the mower onto the driveway and measure from the concrete to the bottom of the blade housing. That is your true cutting height, which is frequently not the number printed on the lever. Most mowers adjust between two and four inches, and on most machines the top setting lands somewhere between 3.25 and 4 inches.

Ontario publishes the grub damage threshold: five to ten grubs per tenth of a square metre, which is roughly a square foot, is where dead patches start appearing. That works out to fifty to a hundred per square metre.

But the threshold is not the interesting part. This is: Ontario states that healthy, actively growing turf will have more roots and can withstand more grub feeding without turf loss, and conversely that a lawn which is not vigorous shows damage quickly after small amounts of feeding. Same number of grubs. Different outcome. The variable is root mass.

Michigan State closes the loop and makes the link explicit: raising the mowing height establishes a grub tolerant lawn because of the larger root mass. Their extension service goes further than we would in print, suggesting that height plus modest fertilizer plus watering in dry spells may remove the need for any pesticide at all.

Chinch bugs follow the same logic. Ontario's threshold is more than twenty chinch bugs per cylinder, or roughly ten found with the bucket method, and the page notes that irrigated lawns tolerate chinch bug feeding better without visible damage. Publication 845 adds that too little water induces dormancy and makes turf more susceptible to both hairy chinch bug and white grub.

None of that is a claim that mowing height controls insects. It is a claim that it changes how much damage a given population does, which is a different and much better supported statement.

Which matters more in some years than others, because the grub population is not a constant. Eggs have to take up water from the soil before they hatch, so a dry July thins them out and a wet one does not, and southern Ontario has just had four consecutive months of above normal rain. We have written the whole of that argument out, with the counting method and the thresholds, in lawn grubs in Oshawa.

Andrew owns the business and sets the deck himself. North Oshawa, with the mowing route reaching all seven Durham towns, fully insured and an HST registrant. Six winters clearing snow in Durham Region are his own, and that deck has sat at one setting on every property, every week, since the first one.

The order on a weekly visit runs edge, then mow, then trim, and it goes that way round because edging first means the mower picks the debris up instead of you.

Height is only half of the question. The other half is when to stop, and that is when to stop cutting grass in Oshawa, the last cut of the season.

Every number above comes from a ministry, an agency, a municipal by-law or a university turf program, and the sources are linked so you can check us.