I’ve been helping some friends learn to garden this year. I absolutely love getting other folks hooked on gardening! Recently someone sent me the following question:
I’m doing pretty well finding info in my Sunset Magazine books and on the Internet. There is one question not in the books and no one seems to agree on the Internet. What are the parameters for when to water? I know I’m not supposed to water in the afternoon on a hot day, but can I water in the afternoon on a cool day? Afternoon on a warm day? How about 6 pm on a warm day?
Watering, ah, always complicated! It’s one of those things you set guidelines for, and then kind of muddle along learning by trial and error. To help things along, let’s talk about the overall theory of watering plants, so that the novice gardener can get a feel for why and how watering is vital, and how plants respond to watering. Armed with that knowledge, you can then reason things out and generate common sense rules for watering.
So, what about plants and water? Not enough water and you get cell damage and stunted growth. Too much water and you deprive roots of the air they need to do their job, and you risk opportunistic infection by molds, mildews, and fungi. What’s a gardener to do? Well, let’s look at what plants do.
Plants transpire, emitting water as vapor through pores (stomata) in their leaves. They do it to cool themselves and also as part of photosynthesis. Plants are capable of closing the pores in their leaves tightly to minimize water loss on hot or windy days. Not only does this limit water loss, it will physically cause their leaves to droop so that they present less of a target for the sun.
The answer is to tailor watering to the overall conditions. When you water a plant, even at the roots without hitting the leaves, the pores on the leaves open up from a physiological response. This is why it can be bad to water on a windy day or a really hot day– you are tricking the plant into leaving its pores too open and ‘panting away’ its water. The leaves can also get physical burns from water droplets on them acting like a magnifying glass in full sun! On the other hand, you also want the water on the plant and at the base of the plant to drain and dry, so watering at night is usually undesirable– it also attracts snails and wildlife to your garden.
The best times to water are in the morning before 10am and in the evening about 2 hours before sunset. The hottest part of the day is over (or not started), it’s ok for the plants to open their little pores for a couple of hours, and the watering will have dried up and things gone back to normal before any conditions of extreme heat or cold.
If you know you have been watering regularly, and see droopy plants on a hot day, *especially* don’t water right then. Some plants, like tomatoes and squashes, are so good at sucking up their pores that their leaves get really really droopy and they look very bad. In the early evening, they should have perked up and look better. If they don’t, then water the heck out of them, including misting the foliage so they can take water directly in via the stomata (pores).
Also remember, especially if you have plants in containers, that the first 1 – 3 inches of the soil should be primarily dry, so that the roots are getting enough air. Too much water in the soil can prevent your plant from getting the building blocks it needs to grow. It can also provide standing water for algae or anaerobic bacteria to grow and damage your plant. So you want the soil about 3 – 5 inches (or more) below the surface to be damp, but not wet.
Always love to hear other folks’ watering tips and observations, please feel free to comment!
Relentless Inquiry 
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