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

You're welcome!

Southern Magnolia, being evergreen, should ideally be watered as-needed as well, since they are vulnerable to winterburn like hollies, cherrylaurel, and a bunch of other broadleaf evergreens. However, if you have to limit watering for practical reasons due to time and cost, which is understandable, a mature magnolia tree that size can be left to fend for itself, and if its roots intercept any areas of the yard that were watered, the tree will at least pick up some of that moisture. Tree roots are not necessarily any deeper than other plants merely based on their size; the vast majority of tree roots grow more horizontally (parallel to the soil surface, spreading outwards, not down) within the top two feet of soil.

Ornamental cherries should be watered if you are able. They are vulnerable to an array of pest and disease issues, particularly when stressed, and while they are simultaneously not tolerant of soggy soil or saturated root conditions, they also can be stressed by drought. Flower buds, which formed weeks or months ago for next spring's blooms, may abort and die off if they dehydrate too much during winter. (Sometimes they are reduced in number by hungry/thirsty birds eating the flower buds in winter or early spring also, but nothing can really be done about that.)

To be sure, hibiscus roots will have less to keep hydrated once the tops die back, but the roots themselves do not want to dry out too much for too long, and they do still need to keep buds/growth points below the surface for next year's regrowth alive. If the roots do get too dry, the plant might not regrow in spring, or will be stunted or smaller in size until it catches up with replacing roots and buds it lost to dehydration.

Fig wood can be injured by winter cold snaps that produce temps. around or below 20 degrees for several days, which can kill branches and eliminate the fruiting potential for that year, even if regrowth appears from the base of the plant. While they are from a climate that is drier overall, they still don't want to get too dry for long periods, and going into a freeze insufficiently hydrated might cause damage.

Miri