Writing about the possible benefits of invasive plants, I had the fear that for most broad-leafed weeds it would be easy to find medicinal and other value, but that for grasses, I might have to pass. Interestingly, grasses have some good sides, too – even a such-maligned, horrible weed as crab grass.
Crab grass (also called “finger grass” because of its spiky inflorescences, or “fonio”, for African plants) are actually several Digitaria species – “Digitaria” again meaning “finger-like”.
Why is crab grass the proverbial weed? It turns out that “crabs” can’t take hold in a well-watered, well-fertilized lawn. But let that lawn be neglected, and develop some bald spots – that’s where the annual crab grass will move in, taking advantage.
A lawn usually consists of perennial grasses that stay green long into fall and often into winter. Crab grass would be fine to be intermingled, if it would not die by the end of summer and will leave a bald spot – especially if you pull it and do not immediately reseed with normal lawn seed. In that bald spot, its many, many seeds can take hold again. Crab grass’ trick is its long germination period: It might die early, but it can germinate basically all year, as long as there is no snow on the ground. Usually, a bald crab grass spot extends thus from season to season, always looking awful in the fall, showing your neighbors that you are a less-than-perfect gardener.
Remedy? Keep your grass healthy, well-fed, well-watered, well-limed, and reseed in fall and spring, so that crab grass seedlings have no chance.
So, what for is this invasive grass good? For cows and other ungulates like deer crab grass is as nutritious as any other grass; even more so, because of its high protein contents. Sub-Saharan Africa people eat the milled crab grass seeds in porridge and bread. The problem with crab grass is that it germinates and ripens its seed willfully throughout the year. Therefore it must be hand-harvested, defying large-scale cultivation. However, early settlers in America purposefully would till a spot in the spring so that crab grass could grow there, for the grazing of the animals later in the year.
Crab grass (like Bermuda grass) is a warm-weather grass. As such, it accumulates less sugar than a perennial grass - it does not intend to stay around for the winter, needing staying power through the winter. That makes crab grass better digestible especially to horses who might be quite sensitive to a high sugar and starch content – which bloats them, causing colic. So, as hay, crab grass is quite desirable.
Crab grass contains non-trivial amounts of magnesium, phosphorus and calcium – important for bone health, and some vitamin A, folate, and retinol; they might account for its use in eye health: Medicinally, crabgrass infusion is said to be helping against cataracts and feebleness. I probably won’t use it exactly for that purpose. But just knowing that crab grass is not poisonous will land it in my garden teas from now on.
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