Is there a rule of thumb for how much water a garden needs each week?
Baltimore County Maryland
As a very general rule of thumb, about an inch of rain a week is sufficient for moisture-retentive soils (such as clay soils). If you water with a sprinkler, the somewhat crude method of setting out a shallow can or other container to catch the water will let you know how long to run the sprinkler until the water is an inch deep. This applies to established turfgrass and most ornamental plantings. Vegetable or fruit crops may need more or less depending on type and stage of growth. Soil type, plant type, and weather conditions play a large role in varying the moisture needs of the garden. More mature plants often need less pampering than younger plants still getting established, though all benefit from regular watering during drought. With few exceptions (such as germinating turfgrass seed), watering should be good soakings with periods of drying in between, rather than frequent but light waterings. A drying-out period discourages disease as well as encourages roots to grow deeper, where moisture won't evaporate as quickly.
Checking soil moisture by hand - generally by feel - is better than watering based off of a schedule or pattern of rainfall. Some rain events, for instance, soak the soil more effectively than others. Typically, soil should be fairly dry a few inches beneath the surface before needing to be watered. Some plants prosper with below-average moisture levels, such as Yucca, Sedums and other succulents, Rosemary, Lavender, and Sage. Other plants adapted to moist conditions may not want to get even lightly dehydrated and may wilt before soil feels very dry. Always check the soil moisture before assuming a wilting plant is dry, however, as this is not a foolproof indicator. Some wilt during high heat only to recover in the evening (like some Hydrangeas and Impatiens); they wilt simply because they cannot replace the moisture loss from their leaves fast enough to stay turgid (sturdy), and in that case adding more water will not help. Others can wilt if they are already too wet, because the roots are dying and unable to supply the leaves with enough moisture because they no longer function. Here too, adding more water will only make the damage worse.