Do try this at home. Half a dozen plants of little bluestem (Schizachyrium scoparium), planted last year, still not established, and wilting from drought. The plants will be left in dish pans full of water for a day or so to see if they revive. (I posted something similar a while ago.)
Even if they revive, which they sort of look like they’re doing, it’s unlikely that these will remain in the garden for very long. I bet I’ve tried a hundred plants of little bluestem here, and all but a few have died of drought; the remaining ones get the regular water for which they were bred.
Because, yes, little bluestem is native to all of the “lower 48” and even though it’s the same species everywhere, a little bluestem from Ohio never grows in xeric habitats, and that’s what I need here. Plants of the Southwest sells seed, I think not collected in Virginia, so that seems to be the way to go. The bluer varieties are pretty, but they hate me.
The late Christopher Lloyd said that there was no such thing as cheating when it came to gardening, but there is such a thing as duplicity, so I’ll mention the Sporobolus wrightii once again, and show what they now look like after I watered them three times. Yes, I watered them. Stood with the hose for three or four minutes directing the water right at the base of the plant. Boring, but effective.
The soil in the front yard here is bone dry gravelly-stony clay gunk that has been worked in quite a bit (dug, not amended), over the native subsoil of decomposed sandstone.
Despite what you may read or hear, the death of plants by “overwatering in clay soil” (ie, driving the oxygen out of the soil) is a thing difficult to accomplish since the amount of water that would be required to get past the first few inches of topsoil is staggering, clay soils presenting considerable resistance to water penetration. It takes a lot of watering to drown plants, though of course plenty of people seem to be driven to accomplish just that.
Likewise, very dry soils are difficult to wet deeply enough for roots to derive benefit. Hence the repeated watering on my part.
The point here is that half an inch of rain, falling in fifteen minutes, isn’t going to have much effect on the roots of these giant creatures, so standing there with the hose on three separate occasions allows the soil to be moistened enough so that later waterings have more effect.
Where such a downpour has a beneficial effect is for plants adapted to growing in high-oxygenated soils, like desert plants. All of the rain goes to the roots, instead of just bouncing off the surface of the soil, or wetting it a few millimeters deep (which is why many cacti that grow in heavy soils have shallow root systems, to be able to grab this water).
Combine this with regular irrigation and you have double if not quadruple the amount of water going to plants that the natural rainfall provides. Hydroponic gardening of desert plants, you might say. Far from drowning, the plants love it.
I know, this is Soil 101, but when you decide to abandon irrigation as a way of growing plants (far different from trying to save them during periods of severe drought), relying on natural rainfall, the vocabulary you use may seem the same, but the way in which the words are applied comes from an entirely different world.