Tony Koski, Extension Turf Specialist
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Rough(stalk) bluegrass (Poa trivialis) will turn purple or
red when it becomes heat-and/or drought-stressed,
and then brown when completely dormant. |
It’s been an interesting year for lawns. The cool wet spring gave us perfect growing conditions for bluegrass and other cool-season grass lawns – and perfect conditions for some diseases we don’t normally see here. Then it turned hot and dry, causing Ascochyta leaf blight to break out in epidemic proportions. I’ve written about yellow lawns and brown spots. Now we are getting calls and emails about red and purple spots in lawns.
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Rough bluegrass turns brown when completely dormant.
While it looks dead, it will revive itself when cooler
weather returns in the fall. |
The red grass people are seeing is rough(stalk) bluegrass (Poa trivialis) reacting to the very hot weather we’ve had the past couple of weeks. This cool-season relative to Kentucky bluegrass likes hot weather even less than our bluegrass lawns do – so it will often go dormant when we get long stretches of 90+ degree weather. Most often it just goes from green to brown, but it sometimes turns a bright red or purple before browning out. These red grass plants may have stopped producing chlorophyll in advance of becoming dormant (a “smart” thing for a plant to do), leaving red pigments behind – giving the plant a red appearance. Or they are producing extra xanthocyanins and carotenoids (red, purple and pink colored pigments) to protect the leaves from excess light (kind of a sunscreen that the plant produces to protect itself). Either way, the Poa triv patches in some lawns (usually in the hottest, sunniest parts of lawns) take on a reddish or purple cast – before turning a dead-appearing brown.
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While it may appear to be dead, dormant Poa triv
will come back from live stolons on the soil surface. You
can find these green stolons when you dig down into
the patches of dormant, brown grass. |
In spite of the dead appearance, the grass is only dormant. When cooler weather returns, it will green up again. Triv will often go dormant when it’s hot, in spite of generous irrigation. It’s not a drought-induced dormancy, but rather a heat-induced one.
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This happy, green Poa triv is growing in a shady spot
just a few feet away from the brown patch pictured
above. It is aggressively and successfully crowding out
the original grass - turf-type tall fescue. |
When you have a lawn full of Poa triv, you either learn to love it (or at least tolerate it), or you kill it and reseed or re-sod. There is no selective herbicide that will eliminate this weedy grass without harming the bluegrass or ryegrass in the lawn. Poa triv spreads easily and quickly by creeping stolons (runners) to form large patches - especially in lawns that are kept on the wet side. Shade favors its growth, but it can grow quite well in full-sun (but is more likely to go dormant in the sunnier parts of lawns). It is an aggressive, smothering grass under optimal conditions (wet, cool, shady), but less aggressive in drier, sunnier lawns.
I’m hoping for some cooler, wetter weather that will give us healthy, GREEN grass again.
