We don't have an exhaustive list of the most drought-vulnerable plant species, but can say that, since you mentioned azalea specifically, they and their relatives (rhododendron, mountain-laurel, blueberry) are comparatively shallow-rooted and are not nearly as drought-tolerant as other species. Boxwoods are also shallow-rooted shrubs, and dogwoods, redbuds, and other trees that appreciate more even levels of soil moisture would best be watered, especially since spring-blooming trees have already developed their flower buds for next year. On the other end of the spectrum, Yucca are fairly indestructible, and Junipers, while not drought-proof, are fairly resistant to drought stress.

In general, any new plants (those planted less than 3 years ago, as they are still establishing), trees and shrubs, and any drought-intolerant perennials (say, our native hibiscus) should be monitored for watering needs during any period of dry weather. If landscape plants have not been watered thus far, given the drought status most of Maryland (including Montgomery County; see linked map) has been experiencing for nearly the entire summer, then there might already have been damage done to the roots, branches, or the buds for next year's foliage or flowers, but time will tell. Even so, it would help to reduce plant stress to water later rather than not at all. Our Watering Trees and Shrubs web page provides guidance, and its tips can apply to vines and perennials as well.

Evergreens need their retained foliage to stay hydrated during winter, or they will suffer from "winterburn" that can kill leaves and sometimes even branches. If you have to prioritize which trees or shrubs to water, choose evergreens first. A drought-damaged or winter-damaged evergreen might not manifest symptoms until months later. For example, we've seen numerous established 'Green Giant' arborvitae around central Maryland dead or having significant dieback they will not recover from, due entirely or in part to the drought this summer.

For any plant monitoring, feel the soil around four to six inches deep (shallower-rooted species can be checked at four inches, others at six). If the soil feels somewhat dry to the touch at that depth, a thorough soaking of the root zone will probably benefit the plant. How long to water will greatly depend on how you're watering and what the water pressure is like. (Holding a hose with a "rain" wand? Using a watering can? Using an oscillating sprinkler? These all apply water at different rates.) Check to see how deeply the irrigation water seeped into the soil, either several hours or the day after you watered. If it's soaked in to re-moisten the soil at the same depth you were checking it before (six inches), then the amount of water you provided was sufficient. If not, then the water volume or time of sprinkler running may need to be increased. Sandy soil low in organic matter will drain and dry faster than soil high in clay. Compacted soil might resist absorbing water when it gets too dry.

When the soil regularly freezes later this winter, watering can slow or stop, depending on what plants are in the garden/yard that are either newer or not tolerant of drying out. Even so, during mild spells and dry conditions, plants can be checked again and watered as needed. Winter air, being drier, plus sunny days or breezy days, can all gradually dry the soil out during winter if snow or rain is lacking. So far, many areas of the state are several inches behind on rainfall, so it will take several rainstorms to catch us up.

Miri