Can I water plants in the heat of the day?

If you really must water in the heat of the day then don't get the water on the leaves of the plants. It can cause scorching as the sunlight is magnified through the water droplets.Water into the base of the plant where the water is going to do most good.
Best time to water is early in the morning as apparently most plants sleep through the night. Like an early morning coffee for us they appreciate a good drink first thing.
In really hot weather, you'll probably have to water twice a day - morning and evening.
Hope this is of help.

My husband favours to water the garden in the evening but it's not unknown for him to water early in the morning either, like most things in the garden if you want to keep it nice you have to find time to work at it.Hel.xxx.

Evening watering is best. Even early morning sun can dry out plants quite quickly.

I water in morning as its cooler & the water wont shock the flower/plant roots :) If i water after a warm day i always use tippead water so it dont mark the leafs or shock any roots :)