Water in Winter when...
- The ground is not frozen - otherwise you are just building ice up in the top few inches of sand/soil/clay
- The day time temperatures will be 50*F or higher for several hours during the day - this will allow time for the water to soak in deeply and drain away from the surface, again helping to prevent ice build up in the top few inches
- The sun is well up, but not after 1 or 2 pm - this has you watering at or right before the warmest part of the day - and has you finishing enough before sunset so that water will have drained away from the surface (and so not turn into surface ice)
- There hasn't been significant rain or snow MELT in several weeks - and the ground is NOT frozen - due to desicating winds and low humidity, plants need about an inch of water every few weeks during the winter
When watering in winter (or any other time, actually), keep water off of trunks, leaves and crowns of plants. If the water should not drain or evaporate before freezing, it can damage your plants. Roots generally extend a ways from the trunk or crown, so if you water within 6 inches of the trunk or crown, your plant should be able to take advantage of the moisture.
Also, don't forget to mulch, mulch, mulch, mulch - this will help conserve your water and your soil and help keep "heaving" to a minimum. This is particularly important for plants and trees that have been in place only a few years and who may not have as extensive or deep a root system as more established ones. And like water, keep mulch 6 inches or so from tree trunks and crowns of plants (unless the plant, like strawberries, should be completed burried in mulch).