In the Community Garden at 83rd & Dunbarton, we use what we call "the one finger test." That means you take your finger and stick it into the garden soil - about one inch down into the soil. The soil down there should feel moist, like a wrung-out sponge. If the soil feels moist, your plants in that location don't need watering today.
The one-finger test means getting your hands into the soil. That means the real dirt, beneath the mulch. This measurement isn't about whether the mulch is wet, nor about whether the interface plane between mulch & soil surface is moist, but the condition of the garden soil 1" into the soil.
For new gardens, soil surface tension can be an issue, particularly with our sandy soils in Westchester. If there is no mulch on the surface of a new garden, often times the sand particles seem to lock together. Water applied on top simply rolls right off, and the underlying plant is gasping for water. Again, the test is the condition 1" down. If water is running off the surface, some tricks to use include (1) using a hand rake to gently break up the surface of the soil; (2) using basins, gardening in a depression, using mini dams or earth forms to hold the water in place long enough so that it can infiltrate; (3) enriching the soil with compost -- preferably coarse, homemade -- which acts like a sponge to hold moisture, plus provides food for the soil organisms that make up the real richness of organic garden soil; (4) applying mulch, to break the soil tension, enrich the soil at the soil/mulch interface, and keep the soil cooler.
In clay soils, for instance in parts of Culver City, surprisingly enough, the solution is again enriching the soil with compost. A rich garden soil has less moisture-regulation issues. In very heavy clay soils, rather than gardening in a depression, for winter gardens you might need to consider using a raised bed. I'm hesitant to mention raised beds because they get tremendous kudos in East Coast and English garden books, but they aren't really right for Southern California. Here, a raised bed means that the block of soil is exposed to baking summer heat and drying air on five sides instead of just one. For sandy soils, raising your beds is not at all recommended (unless you have soil toxin/lead issues, but that's an entirely different topic). Thus the first thing try is really building up that rich, healthy, alive garden soil to see if that helps your moisture-regulation issues.
In new gardens, these issues are compounded by the fact that compost cycles and soil life populations are not yet established. Some portions of a new garden might have received better tilling or more compost than other sections. Thus there is no other alternative than to use the one-finger test many, many times in different spots around the garden. Over time, observation will teach you about the condition of the soil. A hypothetical example: the spot by the fig is consistently dry every time you try it, while the lavendar always seems to be wet. You'll learn, through many observations over time, that on a rushed day you can water that fig and skip that lavendar. Probably both spots will need some chunky high-organic-material compost mixed in.
John Jeavons refers to a "3 second shiny." That means after you water a well-prepared garden bed, you water until the soil has a shiny layer of excess water which disappears within 1/2 to 3 seconds after watering stops. This is a guideline for a well-prepared bed, and might not be applicable for new gardens or places with tough garden soil issues. Again, ongoing applications of chunky high-organic-material compost will eventually gain you a well-prepared bed.
Some plants -- most notably our California natives, our "drought tolerant" or "xeriscape" plants, and most citrus trees (oranges, lemons, and their cousins) -- don't like to be moist all the time. They like a chance to dry out between waterings. Thus if your one-finger test reveals any soil moisture at all for these plants, do not water them. Very often these are the plants we "kill with kindness" by overwatering. You can observe the overwatering conditions in plants like lavendar and sage when they put on excessive amounts of lush new growth with broad leaves and a rich green aesthetic -- that's not normal. Their normal leaf is supposed to be slow-growing, grey-green, and a bit lean-looking. Plants in this category include (but are not limited to) most California natives, most sages including hybrids, lavendar, rosemary, Mediterranean herbs like oregano and thyme, plants of Middle Eastern origin like pomegranates. In the Community Gardens, we are using a set of signs to mark these water-sensitive plants. Citrus trees that are overwatered tend to turn yellow and drop their leaves abundantly. Again, if your one-finger test reveals any soil moisture at all, do not water them.
For all gardens, the maxim is: Water the soil, not the plants. As you learn more about healthy, alive, organic garden soils, you'll discover that the place we need the water is to keep our vast populations of soil organisms happy. The leaves of most plants (short of rain-forest plants, but they aren't appropriate to So Calif anyway), don't do much as far as water uptake. It is the roots that can access the water, with the assistance of the soil organisms. We don't want to drown the soil organisms (thus the wrung-out sponge analogy). Additionally, there are plants -- most notably our squash family, cucumbers, and other fuzzy-leafed plants -- that absolutely hate to get their leaves wet. These plants will rot away if you hose their topsides. Thus you must bend down and apply the water where it needs to go -- at the soil surface.
It is not recommended to wait until your garden plants are droopy and pale to say they need water. Those visual signs indicate an extremely stressed plant, that has been neglected too far. It is very difficult for a stressed plant to regain full vigor -- some can be stunted and never ever recover. In an in-ground garden situation, this phenomenon often results from improper planting techniques followed by abandonment. If the plant (or vegetable 6-pack or plant plug) was rootbound, and it was planted without fluffing out the roots, water will simply run off the exterior of this tightly wound rootball. For trees, a long-term rootbound problem might not be recoverable; you might have to replace the tree. For vegetables, pay attention as you plant them: don't shove a geometric cube into the ground. Instead, gently fluff out the rootball with your fingers before you slide the plant into the soil. Water the transplants immediately, and in those first few days, don't allow them to dry out. Apply the one-finger test many times around new plantings.
In the Community Gardens, we use signs to mark new plantings and newly-planted seedling beds. These spots need watering attention every time a gardener comes to the garden. Seedling beds are particularly challenging in our So Calif sun. Sometimes to sprout seeds, particularly in less-than-optimum soil conditions, you may need to use shadecloth and other sheltering devices to shield them from the searing sun. Another way to manage the water needs of seeds is to sprout them in pots in a more sheltered area, then transplant; all but root vegetables can be handled this way. Carrots are difficult to sprout, particularly if you have a soil-tension problem as described above. Carrots (and onions) need very consistent soil moisture to sprout, thus probably won't work very well unless we're having a stretch of foggy cool overcast weather. Carrots can sometimes be helped along by interplanting with radishes; the lusty radishes sprout easily and heave the soil upward, breaking the crust for the more delicate carrot seeds.
During the winter months, rainfall can have influence on watering patterns. More likely, the cool nighttime temperatures of our winter months have a greater influence. When soils are cool, moisture evaporates slower. Thus during the cool months, your garden demands far less watering. Simultaneously, the possibility of overwatering our natives, xeriscapes, and citrus is far greater. If we have had recent measurable rainfall, this probably means you'll need to decrease your garden watering. (Turn off those automatic sprinklers!) Measurable rainfall can be determined very simply: by putting a bucket or pie plate outdoors in the garden in question. If a given storm puts an inch or so of water in your measuring device, that will affect your garden's watering needs. But because we have erratic rainfall in any So Calif winter, you must become very diligent at using the one-finger test.
For dry summer months, LADWP's watering regulations aren't limitations, they are good gardening sense. Don't water if it's running off the surface (fix the surface problems - see "sandy soils" above). Don't water before 4pm or after 9am because evaporation in the hot sun means your plants aren't getting the benefit of the water you apply. By watering in the evening, your soils and plants have the entirety of the cool evening, relieved from baking summer sun, to soak in the water you apply. If you are watering in the evening and you are experiencing mildew problems (happens a lot with roses), first, be sure you are keeping the leaves and tops of the plants dry. Water the soil, not the plants. Secondly, examine your plant spacing. Is there adequate space between the plants for air to circulate? It might be that you need to lift and resituate an ailing plant. Mildew issues are far less due to evening watering patterns than they are an indicator of other problems in the garden. Thus, follow LADWP recommendations and water as the sun eases from the sky.
In the Community Garden at 83rd & Dunbarton, we have a watering responsibility schedule. If it is your day, it means your day to do the one-finger test and evaluate watering needs. Our volunteer coordinator tends to populate the calendar for 2 or 3 volunteer visits per week in cool moist months, which we bump up to daily volunteers in hot dry weeks. All volunteers are asked to observe garden signage ("new plantings" and "likes dry conditions") as well as using the one-finger test prior to applying the hose. As a volunteer, upon arriving at the site you can make a quick pass around the garden to do all your observations and one-finger testing. You'll then know, later when you have hose in hand, what needs water and what doesn't.
1) use the one-finger test
2) water the soil, not the plants
3) follow LADWP recommendations and water in the evening