When to water
When your soil is looking dry and the foliage is not looking it’s perky best, it’s time to take a closer look to see if it is time to water. A good way to do this is to poke your finger into the soil near the base of a few of your plants to see if you feel any moisture. If it feels dry down there, it is time to water.
As for the best time of day to water, morning is definitely the prime time. By watering before noon you are giving your plant’s leaves plenty of time to dry out, which goes a long way toward preventing a number of plant diseases.
It’s all about the roots
Focus on the roots (not the foliage) when you are watering your plants. Your goal is to thoroughly soak the root zone of those precious tomatoes, cucumbers, and cone flowers when they need it. For annuals, the root zone is roughly the top 6 inches of soil. For perennials, it is roughly the top 12 inches.
The bottom line
But what if you didn’t notice that your veggies were looking a little limp until later in the day or you’re in a hurry today and don’t have time to let the water soak down a full six inches? Don’t sweat it. Just be careful to keep the foliage as dry as possible (late afternoon is not the time for the overhead sprinkler) and try to give the root zone as good a soak as you can in the time you have available. And maybe plan on running more soaker hose or drip irrigation next year :).