Although you would think we get enough rain in Northern Virginia to keep any sort of plant happy, in fact, sometimes you will have to supplement your plants’ water in order to keep them healthy.
There are several situations where rainwater might not be enough. New sod, for instance, needs to be kept consistently moist until it is well established -- it cannot wait three days for the next good downpour.
Trees sometimes do not fare well, especially in dry years or in urban settings. A mature oak can absorb 100 gallons of water per day, and if the groundwater is insufficient, it will wilt. This powerful thirst is not unique to oaks -- all trees need water. The big ones need more than the little ones, and the young ones will sometimes need supplementation because their roots are not yet deep enough to get at the ground water.
Runoff from poor soils can also be a factor. Two inches of rain does no good if it doesn’t soak into the ground. Sometimes the ground is too compacted for it to absorb, or the terrain is steep and the water washes away before it has a chance to soak in.
As you can see, there are many situations in which is necessary to water. Watering by hand is by far the best option, because this enables you to put exactly as much water is needed, right where it will do the most good. A watering wand is the best tool for the job here.
Since it’s not possible sometimes to water by hand, soaker hoses or sprinklers can be used instead.
The rule of thumb is “water deeply and infrequently.” Except for new sod, this works for nearly all plants. The only thing that changes is the definition of “deeply” and “infrequently.” The idea is that you want to thoroughly saturate the root ball until it resembles a dripping sponge, and then allow it to absorb that water over a period of time, and not water again until it is barely damp.
So for your roses this might be every three days, and for your weeping cherry this might be every week or ten days.
Watering like this will encourage the roots to grow down deeply, rather than shallowly, because you will have “taught” the plant that the water is more plentiful the deeper it goes rather than close to the surface.
And of course, don’t forget to add a thick layer of mulch around your plants. This will hold rainwater long enough for it to soak in, and reduce evaporation so you can water less often. Good gardening!