grass profile2026-10-14By The Turf DeskAgrostis capillaris · cool-season
Colonial bentgrass: the 10 mm lawn that needs a reel mower
Generated from this page: thatch banded by tendency, roots to 120 mm, dot pitch set by the 8 mm cutting floor
Colonial bentgrass is the only species here that will hold a genuine 10 to 20 mm cut. It also has the shallowest roots in the set at 80 to 120 mm, the fastest thatch build-up, and a hard requirement for a reel mower. This is a specialist profile, not a default.
The working band is 10 to 20 mm, with a floor of 8 mm. At that height a rotary mower is not an option: the blades shear rather than cut, and a sheared leaf tip browns within a day and opens the plant to dollar spot. A reel mower with six or more blades and a bedknife set to a paper-test gap is mandatory.
Because the cut is short, the cut is frequent. On a 12 mm lawn the one-third rule allows 4 mm of growth between cuts, which at 6 mm a day in spring means cutting every day, or at least every second day. This species is chosen for the look, and the look costs mowing hours.
Root depth is 80 to 120 mm, the shallowest of any species in this profile set. That means the plant can only draw on moisture in the top 120 mm of soil, and in a hot dry week that layer empties in three days. Irrigate in 6 mm sips every second or third day rather than 20 mm once a week; a deep soak below 120 mm is water the roots cannot reach.
Drought tier is 2 out of 5. Colonial bentgrass will not survive a month of dormancy, and it goes off-colour faster than any other cool-season species here.
Budget 10 g of nitrogen per square metre per year, split 35:30:35. An even spread suits a species that grows steadily rather than in flushes. Early-autumn nitrogen is the one that matters most for winter survival, because the 80 mm root system has nothing in reserve.
Thatch tendency is 5 out of 5, the joint highest in the set. The wiry stems and the dense mat of stolons decompose slowly and the low cut means you cannot mulch the problem away. Plan on verticutting twice a year, taking 3 mm each pass, plus a light topdressing of 1 to 2 mm of sand-based material after each verticut.
Colonial bentgrass belongs on a small, contained, high-visibility lawn: a front entrance, a courtyard, a showpiece panel. It does not belong on a family lawn that has to survive a summer of football and a dog.
Where a warm-season species covers the same ground, the honest comparison is hybrid bermudagrass, which also cuts at 15 to 25 mm but roots three times deeper. Colonial bentgrass wins only on winter colour and on sites too far north for bermudagrass to survive.
Can I cut colonial bentgrass with a rotary mower at 25 mm?
You can, and it will look acceptable for one season. But the low end of the band, 10 to 15 mm, is unreachable with a rotary, and the sheared leaf tips invite dollar spot. If a reel mower is not available, run this species at 20 mm and no lower.
How often does bentgrass need verticutting?
Twice a year on a 10 to 15 mm lawn, taking 3 mm per pass, in late spring and early autumn. Skipping it produces a 20 mm thatch mat within two seasons.
Why is my bentgrass patchy after winter?
Usually a shallow root system combined with a dry autumn. Water 6 mm every third day through the autumn establishment window and the stand enters winter with a full root profile.
Four field symptoms, each read back to the parameter that produced it rather than to a product. The last column is the one number that row turns on, with the threshold drawn on it.
Cutting height has been held below the species floor, thinning the stand from the centre of the lawn outwards until the soil is exposed.
Concentrated traffic along a path, at a gate or in front of a goal has worn through the canopy to soil, and the species has no rhizomes to repair it.
Drought dormancy has gone past the survival limit, four to six weeks for cool-season turf and six to eight for warm-season, and the crowns have died.
A fungal patch disease, large patch on warm-season turf or summer patch on cool-season, has killed the crowns in a ring pattern.
Grub or chafer larvae have eaten the roots under the patch, and the turf lifts away from the soil like a carpet when pulled.
A spill of fertiliser, petrol, herbicide or a concentrated dog urine patch has killed the leaf and the crown together.
Raise the cutting height to the species band: 25 to 40 mm for Kentucky bluegrass, 30 to 50 for perennial ryegrass, 50 to 70 for tall fescue, 40 to 60 for fine fescue.
Scarify or core-aerate the bare area to 75 mm depth so the seed has soil contact rather than a compacted surface.
Seed at the species rate: 15 g per square metre of Kentucky bluegrass, 30 of perennial ryegrass, 35 of tall fescue, 20 of fine fescue. Where the area is over 1 square metre, use a blend rather than a single species.
Where the species has no rhizomes, which is tall fescue, fine fescue and St. Augustinegrass, the only repair is seed or plugs. There is no parameter change that will fill the patch.
For a dog urine patch, water 25 mm immediately and leach the salts below the root zone, then reseed. Do not add nitrogen; the patch is already salt-burned.
For grubs, confirm by lifting the turf and finding 10 or more larvae per square metre, then treat; below that threshold the lawn recovers on its own.
Replace the traffic. A worn path worn again and again needs a hard surface or a paving strip, not another reseed.
Pull a handful of turf at the edge of the patch. If it lifts like a carpet with no roots attached, the cause is grubs or a root disease; if it resists, the cause is surface wear or drought.
Look for a ring pattern. Large patch and summer patch kill in expanding rings with a green centre or a green edge, which wear and drought do not produce.
Check the cutting height against the species band; scalping produces a uniform thinning across the whole lawn rather than isolated patches.
Dig a 100 mm deep spadeful at the edge of the patch and check for larvae, white fungal mycelium, or a hard compacted layer at 50 mm.
The surface stays wet: compacted soil, a thatch layer over 15 mm, or a hollow that collects water keeps the top 20 mm saturated for most of the year.
Light has fallen below the compensation point of the grass, usually under four hours of direct sun, so the turf thins and stops competing.
Soil pH has fallen below 5.5, which suppresses the grass and the bacteria that decompose thatch while moss tolerates the acidity.
The cutting height has been held below the species floor for long enough to thin the stand, opening the surface to colonisation.
Nitrogen has been below the species budget for several seasons, or above it, in both cases leaving a stand too weak to close the surface.
Soil has been compacted by traffic to the point where infiltration drops below 5 mm an hour and every rain sits on the surface.
Core-aerate to 75 mm depth at 50 to 100 mm spacing, two passes at right angles, to break the compacted layer and restore infiltration.
Scarify 5 mm in early autumn where thatch exceeds 15 mm, then overseed at the species rate; a thin stand is what moss is colonising.
Raise the cutting height to the top of the species band. In shade that means 60 mm for fine fescue and 70 mm for tall fescue, not 30 mm.
Switch the species to creeping red fescue, shade rating 5 of 5, where direct sun is under four hours. Moss tolerates shade; most lawn grasses do not.
Apply lime only if a soil test reads below pH 5.5, at 200 to 250 g per square metre on a loam, and retest after 12 months. Do not lime on the assumption that moss means acid.
Bring the annual nitrogen to the species budget in a shaded lawn, which is a third less than in full sun: fine fescue 4 to 6 g per square metre, tall fescue 8.
Improve drainage where water stands: a French drain, a surface swale, or the removal of a compaction pan at 150 to 200 mm.
Push a screwdriver into the soil. If it takes two hands and stops at 50 mm, compaction is the driver and aeration is the first action.
Time how long a 100 mm ring filled with water takes to drain. Under 25 mm an hour is compacted; under 5 mm an hour is severely compacted.
Map the sun: note how many hours of direct sun the mossy area receives in midsummer, not in spring. Under four hours, no grass species will win without changing the light.
Test the soil pH. Moss tolerates a wide range, so pH is only sometimes the driver and should never be assumed.
Read the thatch depth with a knife and ruler; above 15 mm the mat is holding the moisture that moss needs.
Nitrogen has been applied above the species budget for several seasons, producing more lignin-rich stem and stolon tissue than the soil can decompose.
Clippings have been bagged for years on a lawn that is now below 10 mm of thatch, removing the nitrogen the microbes need to break the mat down.
Soil pH has drifted below 5.5, collapsing the bacterial population and shifting decomposition to slower fungal pathways.
The lawn is a species with a thatch tendency of 5 out of 5 and has never been scarified.
Irrigation is frequent and shallow, keeping the surface constantly moist and the mat compressed rather than allowing it to dry and fracture.
Scarify 5 mm, in early autumn when soil temperature at 50 mm is between 15 and 21 degrees Celsius, with the tines set to just graze the mat.
Overseed the same day at the full species rate, 10 to 15 g per square metre for Kentucky bluegrass, 25 to 35 for perennial ryegrass, 30 to 40 for tall fescue.
Topdress 1 to 2 mm of sand-loam matched to the root zone, then water 6 to 10 mm a day for ten days.
Cut the annual nitrogen to the species budget: 15 g per square metre for Kentucky bluegrass, 10 for zoysia, 6 for centipedegrass and fine fescues.
Raise soil pH into the species band with 200 to 250 g per square metre of lime on a loam, applied in the following autumn and retested after 12 months.
Switch to mowing at the top of the species band and mulch the clippings, which returns 1 to 2 g of nitrogen per square metre per year.
Cut a 100 mm square flap of turf with a knife, lift it like a hinge and read the brown fibrous layer between the green leaf bases and the dark soil with a ruler. Do it in three places.
Push a screwdriver into the profile; if it stops in the mat before reaching soil, the layer is thick and hydrophobic.
Watch a hose test: water beading and running off a dry lawn instead of soaking in is the signature of a hydrophobic thatch layer.
Check for the crown being lifted above the soil line by parting the grass with both hands; crowns 15 to 20 mm above soil dry out in two days.
15 mm
mat depth
Broadleaf weeds2026-11-01
This row is released with the 2026-11-01 entry, and will carry the same four columns as the others.
