As you may have noticed, we have had a mixture of dry and wet weather this year. All plants, but especially annuals (such as veggies) and freshly planted perennials, are affected by dry weather, because their root systems are not yet large enough to support the plant and do not yet efficiently collect water from surrounding soils. There is a fine balance to be observed when irrigating plants, though: Too much water will wash nutrients away from young roots and may cause rot, while too little or shallow watering will either allow the plant to dehydrate or bring the roots too close to the hot surface of the soil.
In general, vegetable plants would rather have more thorough and less frequent watering. When the entire topsoil layer is saturated at least twice a week, the roots of a vegetable plant will tend to grow downward, towards the damper and cooler soil beneath them. Not only will the root system collect water more efficiently, it will also draw the roots away from the drier and hotter surface of the soil. This promotes a healthier plant growth pattern, reducing the need for external watering when there is a rain and lowering the chances of the root system being attacked by unwanted fungi.
In the case of other young transplants, such as fruit trees, bushes and shrubs, flowering perennials and other ornamental plants, the amount of water will usually vary due to the soil it was planted in, the size of the original root ball and plant, and the type of plant being watered. For example, a young apple tree from a 3-gallon pot transplanted in clay-based soil will require less frequent watering than a group of raspberry starts in the same soil, due to the higher amount of water loss through the leaves (transpiration) in the raspberry starts. It is always best to check the sticker or plant tag that came with the plant for advice first, but as a general rule of thumb, water trees and ornamental shrubs enough to keep the soil 4 to 6” around the root ball (including below it) moist, but not soggy, every other day that it does not rain. Continue this until they are established. For starts such as blueberries, raspberry and blackberries, hydrangeas, and vining flowers and fruits (like clematis), water 4-6” around the root ball every day it does not rain until they are established in the soil.
Common sense applies in this process as well: if the soil around the plant is still about as moist as it should be at day 2, then hold off on watering until it begins to dry again. A good “quick-n-dirty” test for soil moisture is to grab a handful, squeeze it in your hand, and let go. Optimal moisture content is when most of the soil is still in a ball in your hand, but when you touch it, it begins to break apart again.
In vegetables, it usually takes until the first fruit is set and ripening on the plant for the roots to be fully developed. This typically (of course, depending on the kind of plant) takes anywhere between 3 and 6 weeks from the date of transplant. For trees and shrubs (and blue, black and raspberries, etc.) usually takes most of a growing season to accomplish. For example, if a blueberry bush were set out in early May, it would need this kind of watering until the end of August, and maybe longer depending on the timing of rain or heat.
If you have any questions about watering, how long or when to water, or any other concerns, please feel free to contact your County Extension Office.