In dry grasslands it rains for a bit, the seeds sprout, dormant plants recover, everything goes to seed as quickly as possible and then looks dead again.
That’s not what people want when they sow grass. It’s supposed to be green, a nicely dense carpet, soft under foot, and smooth. Not the big clumps of savanna grasses except maybe as decorative accents.
Not many species do that and they’re not necessarily native or even just compatible with the local climate/soil. The weed grasses in your area probably grow perfectly well on their own, but a *lawn* is a way more difficult to maintain monoculture.
Grass is tough once established, and creates a ton of seeds. Once created, the seeds can hang about for weeks or months. They can be knocked down, transported by birds or other animals and buried. It can be available in a pretty consistent and steady supply. When the wet weather does arrive there will be some that grows and some that doesn’t. The old grass that creates the seed doesn’t just die off, and is able to produce more seed, as well as spreading through its roots. So while not all grass seed will survive germination in the wild, it is always given the opportunity to spread and grow.
I don’t think anyone has said this yet, so here’s another aspect: survival rates.
Sea turtles lay ~100 eggs at a time, and only about 1 in 1,000 hatchlings make it to adulthood. That is a successful strategy as far as nature is concerned, because the species still exists. However, we humans aren’t okay with that. We would rather put in effort raising one single ~~hatchling~~ baby (or occasionally more, ofc) as best as we can, to maximize the rate of humans making it to adulthood.
It’s similar with grasses. Grass plants will happily dump millions of seeds into the landscape, and if a few survive to actually germinate and grow, that’s a win. But we human gardeners would rather put in the effort to maximize that rate, buying 10 lbs of seed and seeing 90%+ germinate under perfect conditions, rather than buying 100 lbs of seed and have <10% germinate under poor conditions.
Seeds can be eaten by various critters and deposited with instant fertilizer, or blown by wind, can remain dormant and kept damp by nature long enough to root. It won’t come in as full and lush as a manmade watered and cultivated lawn start (which really do need lot of careful care), but in the wild some seeds will manage to root and make it and with enough time will fill out by spreading out from that start. Most grass spread comes from healthy root systems expanding their territory and filling in space over long periods of time, all those healthy grassland ecosystems that are not man made just took time and patience.
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