I live in Fairfield, California, Western Garden Zone 14. I am having a hard time figuring how much I should water my plants. My drip system is fairly accurate; so if I know the number of gallons per week (during peak watering time) for my plants, I should be able to get it right.
I have been told that a "small shrub" should get about 10 gallons per week, but I don't know whether that applies here or not. Is there any place that I can get better information?
I would like this information for specific plants as well, if possible. Some of the plants I have are: Hydrangeas, Persimmon tree, Crape Myrtle trees, Lemon tree, Jade plants, Cymbidiums, Roses.
The references I've read all give ranges, and even so, the advice must be interpreted for one's specific conditions. Eg. "10 gallons a week" includes all natural sources, such a rainfall and dew (which can be a crucial source of water for some plants), and may be reduced if the soil retains water well (eg, clay), and increased if water drains away rapidly (eg, sand and gravel). It also depends on the shrub: a rose will need more water, a potentilla less. Thus, at best such a reference supplies another set of data points for determining your watering needs. You can find species-specific watering data by googling on each species.
Here are some general considerations that help figure out how much watering you need to do.
Watering depends not only on the Zone and the plants, but also on recent rainfall, overnight temperature and humidity (which determines the amount of dew), soil type (ie, water retention), flow of water underground in your garden, and so on.
What you really need is a way to tell whether or not your plants need watering. That is surprisingly easy.
General rule A): if the top two to four inches of soil are bone- or dusty-dry, most trees and shrubs need water. (One to two inches in your vegetable garden). Many plants (such as hydrangeas) will usually signal their need even before the soil gets this dry, eg, wilting leaves. In most North American climates, even California, it takes two to six days for soil to dry out that much, depending on the soil's water retention ability as much as on day-time temperatures, etc. You can extend that time by proper mulching, BTW, which not only saves water, it also keeps down weeds, and cools the soil.
General rule B): a good soaking when needed is better than repeated light watering. In fact, repeated light watering is very, very bad. It encourages roots to form in the top inch or two of soil, which means that even a brief drought of a week or so will likely kill plants. A good soaking means an 1 inch or so of rain or equivalent (water retaining soil doesn't need as much water as soil that allows water to drain away quickly.) ("Drought" here means bone- or dusty-dry soil four inches deep inches deep or more, persisting for more than a couple of days.)
You will have to translate that "one inch of rain" into watering rates for your system. In most N. American climates, it translates into one or two good soakings a week, maybe three or more in a very dry climate. But in a very dry climate I would not grow plants that need that much water. For trees and shrubs, water in a circle as large as the outermost branches. For large trees, you needn't water in close to the trunk, just under the outer one third or so of the canopy (in fact, most trees will send their roots well beyond the canopy if they need the water. Some species, such as maples, are very invasive underground.)
General rule C: Know which plants are drought resistant, and which are not. Do not plant them together, that way you don't have to water the whole garden every time. If you don't have much natural rainfall in the summer, avoid plants that need a lot of regular watering. Grow plants that like your climate. NB that some plants do not like wet feet: for those, overwatering is as bad as or worse than drought. In fact, what's drought for some plants is just a mild dry spell for others.
General rule D: Keep your vegetables happy.
Our part of mid-northern Ontario, while not quite as hot as California, can be very dry. We usually go two to four weeks at a time with little or no rain, sometimes longer, which native plants can tolerate, but most vegetables, annuals and many non-native perennials cannot. The comments above guide my watering. We have watering restriction during most of the summer, watering is permitted only every other day. So I soak the front garden every four days, say Monday and Friday, same with the back garden, say Wednesday and Sunday.
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