Thuja Green Giant
I have several Thuja saplings for planting and am holding them in my garage. The roots are in moist soil wrapped in clear plastic. How often and how much water should be provided to them? I anticipate planting them mid to late March. Garage is heated to keep temp above freezing.
Allegany County Maryland
It would be best to plant them as soon as possible, especially on mild-weather days when the ground is thawed. The roots will grow at surprisingly cool temperatures, and by planting them you'll give them a couple more months head start to planting in the spring. This will also even out the moisture levels around the roots - which are very prone to drying out if not in enough soil. The plastic, while the same basic material as a plant pot, may not be as permeable to airflow and could encourage fungal growth by not allowing air exchange around the roots. Watering such a setup is challenging as there isn't quite a container to both hold moisture and provide good drainage. The heating of the garage may also dry the air out, which affects how long the rootball will stay moist enough for the roots.
If you can manage to plant them after this cold snap passes, place them where you intend to grow them and make sure they're planted at the proper depth - look for the root flare, if visible on such young plants, and keep this just at soil level. The root flare is the point where the trunk flares-out and transitions into roots; it's most obvious on older plants. Burying roots too deeply is a common cause of plant failure, even years after planting. If you cannot tell where the flare is, look for the first root branch; this will be as good a guess as any when the plants are not in pots.
If you elect to keep them in storage in the garage, there is no schedule to follow on watering - you will have to check them periodically to see if the soil around the roots is drying out. If so, soak it well but allow it to drip-dry before re-wrapping. This could be accomplished with a watering can or a dip in a bucket or wheelbarrow. If the soil is still wet, wait longer. Even so, if you continue to store them for awhile longer, plant them at the earliest opportunity so they are prone to stress and can start establishing. Once planted, mulch well (but keep the trunk free of mulch) to reduce frost-heaving, and remove the excess mulch in spring to about 2-3" deep over the roots. Trees should be checked for water periodically after planting too, but typically our spring weather provides sufficient rain until summer.