Most New Brunswick lawns are cool-season turf, and the right cut depends on the grass underfoot. Tall fescue wants 3 to 3.5 inches. Fine fescues and Kentucky bluegrass do best at 2 to 3 inches. Perennial ryegrass and rough bluegrass can go down to 1.5 to 2.5 inches. If you don’t know what you have, mow at the taller end of the range and let the lawn show you what it tolerates.
The mowing height for New Brunswick lawns
Height by species
Mowing height sets how much leaf the grass keeps to feed itself, and how deep the shade falls on the soil around it. Species differ in how low they can go before they start to thin. That is why one number for the whole province would be a bad answer. The figures below sit within the ranges published by several land-grant extension services, including Clemson, NC State, Purdue and Kansas State.
| Grass | Mowing height |
|---|---|
| Tall Fescue | 3 to 3.5 inches |
| Fine Fescues | 2 to 3 inches |
| Kentucky Bluegrass | 2 to 3 inches |
| Perennial Ryegrass | 1.5 to 2.5 inches |
| Rough Bluegrass | 1.5 to 2.5 inches |
Most home lawns are a blend, and a seed mix often holds three of these species at once. When you can’t tell what is growing, pick the middle of the overlap, roughly 2.5 to 3 inches. That height is safe for four of the five species and only slightly short for the fifth.
Why only cool-season grasses
The five species above are the cool-season grasses, and they are the only ones worth planting here. Warm-season turf grows hardest in summer heat and shuts down, browning, once temperatures fall. In Moncton the average last spring frost is May 23 and the average first fall frost is October 2, so the growing window is short and the shoulder seasons are cold. A warm-season lawn would spend a good part of the year dormant and would be poorly placed to survive a long winter. Cool-season grasses do their best growing in spring and fall, which suits the calendar.
Those frost dates come from Environment and Climate Change Canada’s 1981-2010 normals for the Moncton station. They describe one place, not the province, and northern and upland districts run weeks behind. Neighboring provinces share much of the grass palette, though the details differ: see How High to Mow a Lawn in Prince Edward Island for the island’s version, and How High to Mow a Lawn in Yukon for a far colder case.
Raising the deck when the heat comes
Summer is when mowing height matters most, even though New Brunswick is not a dry place. Environment and Climate Change Canada’s 1981-2010 normals for Moncton A show about 267 mm (10.5 inches) of rain across June, July and August, out of 1201 mm (47.3 inches) over the whole year. August is the driest of the three at 81 mm (3.18 inches), and June and July run a little wetter. Note the period: US figures on this site use 1991-2020 normals, so you can’t compare the two sets directly.
That total looks comfortable, but it doesn’t tell you how often to water. Soil decides what an inch of rain does. On sandy ground it drains through within the day, so a lawn can sit in a wet summer and still look stressed by the second afternoon. On clay it can hold moisture for a week. Heat is the second variable. Days at or above 90°F pull water out of the soil and the leaf faster than normal, and a single summer thunderstorm often runs off a hard surface instead of soaking in. A cheap rain gauge on your own lawn tells you more than any regional table: two weeks of readings show whether you are getting an inch a week or half of that.
Mowing taller is the practical response. Move to the top of your species’ range when hot, dry weather sets in: 3.5 inches for tall fescue, 3 inches for fine fescues and Kentucky bluegrass, 2.5 inches for ryegrass and rough bluegrass. The extra blade shades the crown and the soil, slows evaporation, and supports a deeper root system. Weeds that need light to germinate find a thick canopy harder going.
The one-third rule, and why it is not arbitrary
Never remove more than one-third of the blade in a single mowing. The rule is old and often repeated, which makes people suspect it is folklore. It is not. Grass makes its food in the leaf, and a heavy cut strips a large share of that factory at once. The plant answers by pulling energy from its roots to regrow leaf, so the root system stalls while the top recovers. Repeat that every week and the lawn gets shallower and weaker.
The arithmetic is simple, and it tells you when to mow. If your height is 3 inches, cut when the grass reaches 4.5 inches. At 2 inches, cut at 3. At 3.5 inches for tall fescue, wait until it reaches about 5.25 inches, and take off 1.75 inches. A lawn on a rainy week in June can pass those marks in five days. In August’s drier weather it may take ten.
That is why a fixed Saturday schedule can let you down. Mow by the height of the grass rather than by the calendar. A tape measure held against the blade takes ten seconds, and it protects you from leaving a lawn for two weeks and then cutting it to its target height in one go. If you do get behind, lower the deck in stages: cut a third, wait a few days, then cut again.
The rule also pays off at the bin. Clippings from a correct cut are short and drop into the turf, breaking down fast. Clippings from a heavy cut clump on the surface and smother the grass underneath. For more on technique and equipment, see Mowing a Lawn.
The first and last cut of the year
The first cut should follow the grass, not the calendar. Moncton’s average last spring frost is May 23, but that is a midpoint, not a safe date. In one year out of two the last frost comes later, and districts to the north or at elevation run behind that. Wait until the lawn is actively growing and has reached the height where the one-third rule lets you mow without scalping it. Set the deck to the top of your species’ range for that first pass. A lawn coming out of winter has less in reserve than one in July.
Don’t cut short in spring to look tidy. Fine fescues and bluegrass that start the season at 2.5 to 3 inches fill in faster than those scalped to the bare minimum. Rake out any matted clumps of old grass before the first mow, and keep the blades sharp, since a dull edge tears the leaf.
The last cut of the year follows the same logic in reverse. Moncton’s average first fall frost is October 2, the same midpoint caveat applies, and half of years run later. Growth does not stop on the day of the frost. Cool-season grasses keep growing on mild fall days, and you keep mowing for as long as the grass does. Taper the height gradually, and on the final one or two passes, step down toward the lower end of your species’ range, but not below it. Leaving the lawn very long invites matted grass under winter snow, and cutting it too short exposes the crown.
Keep a note of the dates you actually mow first and last each year, since your own yard’s record is the one that matters. For regional agronomy questions beyond what is covered here, Agriculture and Agri-Food Canada is the federal starting point.
Common questions
What is the best mowing height for a New Brunswick lawn?
It depends on the species. Tall fescue does best at 3 to 3.5 inches, fine fescues and Kentucky bluegrass at 2 to 3 inches, and perennial ryegrass and rough bluegrass at 1.5 to 2.5 inches. For a mixed lawn of unknown makeup, 2.5 to 3 inches is a safe working height.
Should I mow shorter in spring to help the lawn green up?
No. A short cut doesn’t speed up growth, and a lawn just emerging from winter has little in reserve. Start at the top of the species range, mow when the grass has grown enough to satisfy the one-third rule, and lower the deck gradually only if you wish to.
Does plenty of summer rain mean I don’t need to water?
Not necessarily. Moncton A averages about 267 mm (10.5 inches) from June through August, with August the driest month at 81 mm (3.18 inches). But sandy soil drains within the day, thunderstorm rain can run off, and heat above 90°F dries the soil quickly. A rain gauge on your own lawn will tell you more than a regional average.
When should I stop mowing in the fall?
When the grass stops growing. The average first fall frost at Moncton is October 2, but that is only a midpoint and in half of years it comes later. Keep cutting while there is growth to cut, lower the deck toward the low end of your species’ range for the last pass, and don’t go below it.