Mature emerald cypress trees, or Thuja occidentalis, are moderately drought tolerant. Even so, there will be times when even well-established specimens may need some supplementary watering. New plantings and very young trees require adequate moisture, especially throughout their first year. During the growing season, natural rainfall may not provide enough moisture for this plant. This is especially important for younger trees, which haven't yet extended their root systems beyond the natural drip line, so the foliage tends to block rainfall. Don't wait for foliage to turn brown before you water, as it may be too late by that time.
Poke your finger deeply into the soil 2 to 4 inches away from the emerald cypress tree's root ball. If the top 2 to 3 inches of soil feels dry, it's time to water.
Water slowly and deeply for about 30 minutes. Allow the water to soak into the soil completely.
Check the soil again. If the top 2 to 3 inches isn't evenly moist, water for another 20 to 30 minutes. This may take as long as an hour or two, depending upon how dry the conditions are. Keep watering and checking until the top 2 or 3 inches of the soil are moist.
Mulch the planting area with 2 to 4 inches of organic compost. Water enough to evenly moisten the surface.
Allow the soil to dry out slightly. Water only when the top 2 or 3 inches of soil feels dry to your touch throughout the growing season.
Reduce watering frequency during the winter months. It's not much of an issue when the plant is dormant. Let the top 3 or 4 inches of soil dry out completely before watering until spring. Weather is typically wet enough from November through March to keep the emerald cypress well supplied.