Watering in winter needs to be done (as-needed, based on soil dryness) when the soil is thawed, as frozen soil will not absorb water readily, if at all. This is why it's important to check on young plants and evergreens that are less tolerant of dry spells in winter when there are stretches of mild weather and little rain/snow, especially before a forecast deep freeze that is likely to keep the soil surface frozen for several days.

Air temperatures fluctuate more rapidly than soil temperatures, and soil is a good insulator, so the further below the soil surface you go, the less impacted that soil is by surface air temperatures. Maryland soils typically do not freeze very deeply in an average winter; while the top inch or so might freeze in a cold snap, the soil several inches down is usually not frozen. Like above-ground plant parts, roots have a natural antifreeze in the plant cells that prevent cold damage (to a point...every plant has its cold hardiness limits). This safeguard works best when the plant is not drought-stressed by too little soil moisture.

The same amount of water is needed as when you would be watering in summer, it's just the frequency of watering overall that would be reduced in winter, since the soil isn't drying out as often or as rapidly. So, for example, a mid-sized shrub might need around 5 gallons of water per watering, whenever the soil becomes somewhat dry to the touch about six inches deep next to the roots. The same guidelines for watering (check the soil moisture by touch or sight, at that depth) apply, it's just that you'll need to water far less often in winter than in summer.

An exception to watering frequency would be any plant kept in a container over the winter. Potted plants outdoors do not have the benefit of insulating soil around their roots (beyond the limited protection the container's size gives it, which is minimal), so the soil will dry out fairly quickly and it will also change temperature in freeze/thaw cycles more readily than plants in the ground, adding to plant stress and potentially dehydration and root death. Shrub and tree roots are not as cold-hardy as a plant's top growth, since they don't need to be (in the ground, they are far more insulated than the branches and any evergreen foliage), so that disadvantage makes potted plants more at-risk of serious damage during winter, even if they are reliably cold-hardy in the ground.

Mulch depth doesn't need to change either from summertime levels. A depth of about 3 inches is fine, and only very particular plants known to need extra protection due to their borderline hardiness (like Hardy Banana, Musa basjoo) may benefit from a thicker layer. The risk of applying too much mulch is that it can impede moisture percolation into the soil, and it might also hamper how much oxygen reaches roots, which is necessary to keep them alive and healthy.

Miri

Thank you, Miri.

You're welcome.