When is the best time to water my plants?

While it's always good to plan wisely about watering your lawn and garden, I find it particularly important in times of drought, when our plants rely on us for hydration.

The best time to water -- be it hand-watering or using an irrigation system -- is in the early morning, before the temperature gets warm. During this time, the air is still cool, and therefore you won't lose a lot of water to evaporation. More water is penetrating the soil and not being wasted, which is especially important if you're using a sprinkler system that sends water droplets through the air before hitting the soil where the moisture is actually needed. This also gives plants the water they need throughout the day, and won't have to spend the first part of it dry and wilting. As the temperature gets hotter, the water is already available and the plants are better able to combat the heat. If any water got on the leaves, it has the whole day to dry out before the air gets cool again at night.

If you have an irrigation system with a timer, watering at the correct time of day is a no-brainer. Set it for the early morning, and you're good to go. (Though you should still look below for some other water-savings tips.)

However, without automatic irrigation, it's not always possible to water at the preferred time. What are some second-best solutions?

Particularly if we're in a drought situation, the honest answer is that watering any time is better than skipping watering altogether because it isn't the best time of day (but I'd make a case against the lawn -- see below). If late afternoon or early evening is the only time available, then it's better than nothing when your plants are thirsty. However, there are a few things to keep in mind:

Please, stop watering the air!

• Sprinklers: Water the soil, not the air! This rule applies no matter what time of day you water, but it's especially important when it's hot out. The roots are the part of the plant that needs water, so your goal should always be to get the most water into the soil. Realize that when you use a sprinkler system during the hot afternoon sun, a significant percentage of that water simply evaporates, and another portion of it just wets the leaves and doesn't get to the roots. This will vary depending on the type of sprinkler head. Some release a fine mist, which means a higher evaporation rate. Others are a little more water efficient, spraying larger droplets to reduce evaporation. Nonetheless, it's still unwise to use sprinklers at the hottest time of day, as there will always be some amount of evaporation happening. 
• Hand watering: Again, water close to the soil. The same idea goes for using the hose or watering can. Keep the water as close to the soil as possible. Many hose nozzles have different settings, from powerful jet to fine mist. If it's hot and you're spraying a wide area, just remember that any water droplets traveling through the air will partly be lost to evaporation. It takes a little more effort to move the hose closer to the plant roots, but less water will be wasted.
• Keep the leaves dry. If watering later in the day, avoid getting the leaves wet, as there won't be enough heat left for this water to evaporate. Cool, moist leaves will encourage fungal problems, such as powdery mildew.
• Let the lawn go brown. I know this can be difficult for many people who have a hard time accepting a less-than-perfect lawn. But the truth is, keeping the lawn green during drought months requires a lot of water, and it's worth it to evaluate how necessary this is, and lower your tolerance. The grass will go dormant, but it's not going to die. If your area has a particularly long stretch without rainfall, you can give it some water to keep it alive, but don't overdo it. Every year water is becoming a scarcer resource (just look at what's already happened in California and in other parts of the world), and it's time to reconsider the importance of a perfect green during the summer months.
• Avoid installing new plants during the summer. New plantings require a lot of TLC. Regular watering is necessary during the first couple of years to get your plants (even the "drought-tolerant" ones) established, and this is even more challenging during long, hot, dry days. In the spring and fall, when rain is abundant, it will be much easier to get these plants going without stressing them out during the hottest months.
• Mulch. In your garden beds, use a top layer of mulch to retain moisture.
• Check for leaks. Evaluate your plumbing, hoses, and irrigation equipment for leaks, and make repairs as soon as possible.

No one wants to waste water. It's an urgent environmental issue, but also a personal one. Whether you buy your water from the city or have your own well, you don't want to waste your money or finite water supply on inefficient irrigation.