It is hard to keep from crying when the landscape turns crisp around us and no amount of water is enough to replace what the plants lose to blistering heat.
Do all you can to keep it moist if:
You don't have to water the whole yard or a big area.
Pour a bucket of water slowly over the plant's root zone
Let a hose drip there for an hour or two.
Cover the soil with mulch, if it's bare.
Set a full but leaky bucket or jug firmly onto the soil.
Up-end long necked bottles filled with water to seep into the root zone. (More in Growing Concerns 311.)
(See What's Coming Up 151 for details.)
If it's in permanent wilt and won't "come to" no matter how much or when you water, lay it to rest. When plants are severely stressed, all manner of pathogens and pests home in on the weaklings' distress signals. Don't make matters worse for the survivors by allowing a build up of infected or infested material.
Right: This plant has had it. When it has been too often left dry and hot, cool moisture loving Hydrangea develops a chronic wilt that is likely the root rotting disease called armillaria. When this happens no amount of water can bring it around. The owner reportsthe plant has been wilting regularly since before summer went dry, so it was a goner from the get-go.
Below, right: It probably didn't help the plant when, several weeks ago, chemicals meant for the lawn drfited onto the leaf. White residue remains to tell the tale.
Plants should never be sprayed with pesticides when it is very hot and dry, as most such solutions have an oil base that can burn foliage under those conditions.