· by Michael Blair, Nashville's Irrigation
Fall overseeding is the most important lawn job of the year in Middle Tennessee, and most of the seed that fails does so for one reason: the irrigation controller was left on its summer program. Germinating seed and established turf want almost opposite watering, and the schedule has to change as the seedlings grow. Three phases, each with its own rule, take the lawn from sowing to the first mow.
Phase one: germination
Until the seed sprouts, the top half inch of soil has to stay damp continuously. Not wet, damp. That means short, frequent cycles: a few minutes per zone, two or three times a day, timed to keep the surface from drying between them. A deep soak once a day does not work because the surface dries by afternoon and a seed that has begun to germinate and then dries out is dead.
Tall fescue takes roughly a week to two weeks to show, depending on soil temperature, and in a warm September it is toward the short end. Watch for the green haze, and keep the light frequent cycles going until it is there across the whole lawn, not just the shady patches that stay damp on their own.
Phase two: seedlings
Once the seedlings are up, the roots are shallow and the plant is fragile, but it no longer needs the surface kept damp. Move to once a day, a little longer per zone, in the early morning. The aim is to wet the top inch or two, where the new roots are, and let the surface dry between waterings so the seedlings are not sitting in a film of water that breeds damping-off fungus.
This is the phase where too much water does the damage. Puddling, a slick surface, seedlings that flop over and turn brown at the base, are all signs of overwatering. Back off a day if the ground stays wet.
- Germination: light cycles two or three times a day, surface always damp
- Seedlings: once a day, early morning, top inch or two
- Establishing: every two or three days, deeper
- After the first two or three mows: back to the normal schedule
Phase three: establishing
When the new grass is a couple of inches tall and has been mowed once or twice, stretch the interval to every two or three days and lengthen the run so water reaches deeper. Roots follow water down, and this is the point where the lawn either develops the deep roots that carry it through next July or stays shallow and dependent.
By late October the nights are cool and evaporation is low, and the lawn can go to the normal fall schedule, which in most years here is little more than what rain provides. Turn the controller down rather than leaving it on a September program into November, when the ground stays wet and the fungal problems start.
Coverage, and the patches that fail anyway
An overseed shows up every weakness in an irrigation system, because the seed has no roots to reach for water that lands a foot away. Bare strips that follow the edge of a spray pattern, or the side of the lawn that a head no longer reaches, are the system telling you something a mature lawn was hiding. Run each zone and watch it before sowing rather than after.
Slopes shed water faster than it soaks in, and the top of a slope dries first. Those areas need shorter, more frequent cycles than the flat, which is an argument for putting them on their own zone if the system allows it. Where it does not, a light covering of straw holds moisture long enough for the seed to catch.