Blog - Weeds in Gardens

Short answer: grass seed will not establish on compacted soil until you open it up, and the fix depends on how deep the hard layer goes. If a screwdriver goes 4 in (10 cm) into moist soil with a firm shove, two or three passes with a core aerator plus a thin compost top-dressing is enough. If it will not go in at all, or water puddles for hours after rain, till 4 to 6 in (10 to 15 cm) deep, work in 1 to 2 in (2.5 to 5 cm) of compost, firm, then seed. Gypsum and sand will not loosen ordinary clay; only organic matter and physical loosening do. Seed at the full rate, water lightly several times a day until germination, and keep traffic off the soil while it is wet afterwards. Why seed fails on hard soil Seed needs moisture, soil contact and somewhere for the root to go; compaction removes the last two. The UMass Extension turf program lists the results as lost pore space, lower soil oxygen, slower infiltration and higher soil strength that limits rooting depth, and notes that weeds such as annual bluegrass and goosegrass thrive where turf cannot. Hard, crusted clay: seed dropped on a surface like this has no loose soil to root into. Photo: "Cracked earth" by CIAT International Center for Tropical Agriculture, CC BY-SA 2.0. Seed scattered on a crust sits on top, dries out between waterings or washes into the low spots, and seed that does germinate sends a root down a few millimetres and stops. Penn State Extension puts a number on it: root penetration falls steadily as soil resistance rises, and almost no roots enter soil at 300 psi. A patch that greens up for two weeks and then thins in the first dry spell is the classic sign that roots never got below the surface. How to confirm the soil is compacted Dry clay is hard by nature and softens after rain, so test when the ground is moist, ideally a day after a good soak. Water standing on a lawn after rain is a common sign that compaction has closed the soil pores. Photo: "Soggy Lawns" by mikecogh, CC BY-SA 2.0. Screwdriver test. Push a long flat-blade screwdriver into moist soil. Clemson Extension reads it this way: little resistance means aeration is not needed; difficulty pushing it in means it probably is. Penetrometer. Penn State recommends testing about 24 hours after a soaking rain, pushing at roughly 1 in per second and noting the depths where the reading first passes 300 psi and where it drops below again. That tells you how thick the hard layer is, which decides between aerating and tilling. Puddling and runoff. Water standing more than an hour after moderate rain, or running off a gentle slope, points to lost pore space. Bare, hard spots in a pattern. Compaction follows traffic: the path to the shed, the strip where cars park, under the swing, wherever a builder's machines sat. If the soil passes these tests and grass still refuses to grow, the problem is something else; our guide to why grass will not grow covers the other causes. Your options, ranked by effort MethodWhat it doesBest forEffort Core aeration, 2 to 4 passesPulls plugs 2 to 3 in deep; 20 to 40 holes per sq ftThin lawn on moderately compacted soil; overseedingLow: rent a machine for half a day Vertical mowing or slit seedingCuts grooves 1/4 to 1/2 in deep and drops seed into themSurface crust, thatch, poor seed contactLow to medium Aeration plus compost top-dressingFills holes with organic matter so channels stay openClay lawns you want to improve without starting overMedium: about 0.8 cu yd compost per 1,000 sq ft per 1/4 in Till 4 to 6 in and work in 1 to 2 in of compostBreaks the compacted layer and changes the topsoil for goodBare ground, builder's fill, soil a screwdriver will not enterHigh: full re-seeding job Build up with imported topsoilAdds a rooting layer above a base that cannot be tilledShallow soil over rock, rubble or rootsHigh and expensive Core aeration: moderate compaction, some grass present A hollow-tine core aerator removes soil rather than pushing it aside; Clemson's specification is tines 1/4 to 3/4 in wide, 2 to 3 in apart, reaching 2 to 3 in (5 to 7.5 cm) deep. One pass is not enough. Iowa State University Extension recommends three or four passes when renovating, to reach 20 to 40 holes per square foot, and Clemson suggests six to ten passes over sparse areas you intend to reseed. Cross the passes. Aerate when the soil is moderately moist so tines penetrate but cores do not smear; if soil sticks to your shoes, wait (more on that in should you aerate after rain). Seed immediately so the holes become seed pockets, then drag a mat or rake over the area to crumble the plugs. Spike aerators do not remove soil and give little lasting benefit on compacted ground. Vertical mowing or slit seeding: crust and thatch A vertical mower or slit seeder cuts narrow grooves in the surface. Iowa State says one or two passes reaching the upper 1/4 to 1/2 in of soil are sufficient, and a slit seeder drops seed directly into the slits. This fixes a crust or thatch layer keeping seed off the soil, but it does not reach a compacted band 2 or 3 in down, so contractors often core aerate first and slit seed second. Check when dethatching actually helps before renting one. Aeration plus top-dressing: clay you want to improve over time Filling aeration holes with compost keeps the channels open and raises organic matter in the root zone. Virginia Tech's cool-season lawn renovation guide notes that as little as 1/4 in of quality compost over a seeded area improves establishment; a 1/4 in (6 mm) layer takes about 0.8 cubic yards per 1,000 sq ft (0.6 m³ per 93 m²). Spread it after aerating and seeding, drag it in, and repeat each autumn. Till and amend: bare ground, builder's fill, no penetration When a screwdriver will not enter moist soil, or you are seeding bare dirt left after construction, you need to rebuild the top layer. The University of Maryland Extension method is to spread 1 to 2 in (2.5 to 5 cm) of compost, which is 3 to 6 cubic yards per 1,000 sq ft, and till it into the top 4 to 6 in (10 to 15 cm). Oregon State University's establishment guide goes to 6 to 8 in where the machine allows, and warns against exceeding about 15 percent organic matter by volume because the soil settles unevenly as it decomposes. Strip existing vegetation and remove stones larger than a golf ball. Wait until the soil is moist but not wet; tilling wet clay smears it into clods that dry like bricks. Spread the compost and make one or two tiller passes as deep as the machine goes, crossing directions on the second. Grade so water runs away from buildings, then firm with a light roller. The Utah State University standard is that footprints and spreader wheels should not leave deep prints or ruts. Let the bed sit several days to two weeks so fresh compost settles and salts leach, then rake lightly and seed. The full new-lawn procedure, including soil testing and starter fertiliser, is in how to prepare soil for grass seed; this article covers only what changes when the soil is compacted. Break up or build up? Till when the hard layer is within reach of the machine and there is real soil underneath. Build up with imported topsoil when you hit rock, rubble, a concrete-hard pan or tree roots within the top 6 in. A rooting layer needs at least 4 in (10 cm), preferably 6. Scarify the top inch of the hard base first so the layers knit; a sharp boundary between loose fill and hard subsoil holds water like a bathtub and roots stop at the line. Oregon State notes that sandy soil placed over clay should be a layer at least 6 in deep, not tilled in. The gypsum and sand myths Gypsum (calcium sulphate) improves structure only in sodic soils, where excess sodium disperses the clay and calcium lets it clump again; those soils occur mainly in arid and some coastal regions. Iowa State University Extension states that despite advertising claims gypsum is of little benefit on their clay soils and is chiefly used to amend sodic soils in the western United States; for clay lawns their advice is core aeration, and for beds it is organic matter worked into the top 8 to 12 in. Washington State University's review adds that gypsum's effects last months rather than years, that heavy use can induce magnesium, iron and manganese deficiencies, and that a soil test should come before any application. Sand makes clay worse unless you add an enormous amount. The University of Arkansas Extension explains that clay particles fill the spaces between sand grains and act as a glue, giving a denser soil, and that changing the texture would take 50 percent or more sand by volume, which is neither realistic nor economical. A thin sand top-dressing to level a lawn is a different practice and does not fix compaction below it. Seeding rates and depth Use the full establishment rate and make sure seed touches soil. Rates below are from the Maryland and Oregon State publications; follow the label if it gives one. SpeciesNew lawn, lb per 1,000 sq ftApprox. g per m² Tall fescue6 to 8 (up to 12 in the Pacific Northwest)30 to 40 Kentucky bluegrass2 to 410 to 20 Perennial ryegrass5 to 1025 to 50 Fine fescues4 to 6 (up to 10)20 to 30 Bermudagrass (hulled)15 Zoysia15 Split the total in two and spread the halves at right angles. Virginia Tech's rule of thumb for tall fescue on bare ground is 10 to 15 seeds per square inch; one seed per square inch is too thin. Rake lightly so seed sits within the top 1/4 in (6 mm), or cover with 1/4 to 1/2 in of screened compost as Maryland suggests, then roll once with an empty roller. When overseeding a lawn that is still more than half grass, cut the rate by about half, as Virginia Tech advises for partial renovation. Sow cool-season grasses from mid-August to mid-October and warm-season grasses in late spring once soil is warm. Watering for germination on tight soil Tight soils take water slowly, so long soakings run off. Oregon State's guidance is two to four light waterings daily at first, just until the surface glistens, which on heavy soil is often 5 to 15 minutes per zone, reducing gradually over four weeks; Maryland's figure is about 1/4 in (6 mm) per session. Expect perennial ryegrass in 5 to 10 days at good temperatures, tall fescue in one to two weeks and Kentucky bluegrass in two to three. Once seedlings are up, shift to fewer, deeper waterings so that by six weeks the soil is moist 4 to 6 in down; that trains roots through the layer you opened. Puddles forming during watering mean a hard pan is still there and the area should be cored again next season. Common mistakes on hard soil Scattering seed on the crust and hoping. Without contact and a way in, most of it feeds birds or washes to the low spots. If you are seeding into an existing lawn, deal with weeds first so you are not opening the soil for them. One pass with a rented aerator. A single pass leaves well under the 20 to 40 holes per square foot you need. Budget the extra half hour. Gypsum, sand or "clay breaker" products instead of organic matter and physical loosening. Preventing re-compaction Compaction returns from the traffic that caused it, and moist soil compacts far more easily than dry soil. Stay off new seed until after the third mowing; never mow, roll or park on the lawn when the soil takes a footprint; route regular foot traffic onto stepping stones or gravel; and keep vehicles and material piles on hard standing. A compost top-dressing every autumn feeds the organisms that build pore structure. On a clay lawn with normal traffic, core aerate every one to two years in the season your grass grows fastest: autumn for cool-season grasses, with at least four weeks of growing weather left, and late spring to early summer for warm-season grasses. UMass reports rooting improves within two to three weeks of coring. A worn shortcut path: repeated foot traffic packs the soil until grass can no longer grow. Photo: "Desire path" by wetwebwork, CC BY 2.0. When to expect results The real test is the first dry spell. Grass seeded into properly loosened soil stays green through a week without rain; grass seeded onto a crust browns off in two or three days. If the new lawn thins in its first summer despite watering, push the screwdriver in where it failed. A hard layer at 2 in means aerate harder next autumn; a hard layer at 4 to 6 in means the tiller did not reach it and you are now improving the soil over time with annual coring and compost. Sources Clemson University Home and Garden Information Center – Aerating Lawns UMass Extension – Compaction and Cultivation Penn State Extension – Diagnosing Soil Compaction Using a Penetrometer Iowa State University Extension – Overseeding a Lawn Iowa State University Extension – Will an application of gypsum improve a clay soil? Washington State University – The Myth of Gypsum Magic (Chalker-Scott) University of Arkansas Extension – Will adding sand help my clay soil? University of Maryland Extension – Starting a New Lawn Oregon State University Extension – Practical Lawn Establishment and Renovation (EC 1550) Virginia Cooperative Extension – Cool Season Lawn Renovation (SPES-353)