Author: Mary Petres

Hello fellow gardeners, we are deep in the throes of summer in RVA and we wanted to take a second to cover some basics on watering! It sounds easy enough, you set the irrigation system or turn on the sprinkler and walk away, right?  Unfortunately, it’s not as easy as that, especially when working with young or newly transplanted plants. 

Because there is no hard and fast watering rule that applies to all plants, it’s quite difficult to give blanket watering advice.  As a rule of thumb though, during the first two years after planting shrubs, trees, perennials, vines, and grasses should be watered twice weekly from March through mid-December if Mother Nature is not delivering 1”- 1  ½” of rain per week. During severe heat (80 degrees and above on a consistent basis, you may need to water 3x per week). For the winter months of January and February it is recommended to water 1-2x per month if we are not consistently getting 1” of rain within a two week period. Alternatively, a foot of snow provides 1’’ of water as it melts.

Plants are extremely variable and have different requirements to thrive. For example, evergreens are particularly susceptible to the desiccating effect of cold wind and temperatures, and often require more watering when it’s windy and cold. This is especially true for fall plantings.

When the valve is fully open, most hoses produce 4 gallons per minute if no breaker (water nozzle) is attached.  However, you generally want to deliver the water at a slower rate than that to allow the water to soak into the ground instead of running off.  You also don’t want to wash the soil away from the base of the plant. Not only will this result in wasted water and a high water bill, but the plants aren’t able to properly absorb the water before it runs off. 

A good starting point is 30 seconds per perennial, 2 minutes per shrub, and 5-6 minutes per tree.  If the water is puddling or running off the surface of the root ball you may need to break up the watering into smaller increments to give it a chance to absorb.  Remember to water all the way around the plant otherwise the water will not wick through the root ball from one side to the other which can cause part of or even the entire plant to die. 

If using a sprinkler system to water the plants, use a rain gauge or 4 oz tuna can to measure how much water the heads are delivering with a targeted goal of ½” of water per watering. Just like with manual watering, you want to be sure the irrigation system is delivering water to all sides of the plant. As the plants grow, heads often need to be adjusted or moved to continue to be effective. And remember to turn your irrigation off during the winter, and winterize the system, to prevent pipes and/or the back flow valve from bursting. 

Generally, your watering approach for evergreens will need to be unique. Because evergreens have more foliage than deciduous trees at an earlier age, the canopy prevents rain from penetrating through the branches to the root ball as easily. For the first year after planting, you may need to water with a hose directly at the root ball to ensure that the evergreen is getting adequate water. If you add additional plantings at the base of evergreen trees, they too may need additional hand watering because the evergreen can make it difficult for rain to hydrate the soil beneath the tree’s branches. The same is true when adding plants beneath deciduous trees that have a large canopy.

If you aren’t sure how dry the soil is around your plants, you can run a simple moisture test to get a good idea of how much water is being retained for use by plants and for how long. 

Moisture Test: Stick a sharp object 8-10 inches in the ground 2-4” outside the root ball and also through the root ball.  If soil sticks to the probe you have watered long enough and/or the soil has enough moisture.  Trees and shrubs need to be watered deeply to encourage proper root development, but be careful that you do not water too much.  The ground should never stay sopping wet or the roots of the new plants will rot.

Alternatively you can also use a moisture meter.  They are available online as well as at local nurseries and shops and can often be found for less than $15.  Remember that you need to test around each type of plant.  Some plants may need more or less water than others and every site has different soil and light conditions.  Testing before watering is always recommended.  During the first year after planting, test immediately beside the root ball.  In year two, you can move out from the root ball 2-4”. 

Perennials and vines may need to be watered more often than other plantings. That’s because the root ball is generally smaller and the mature root system is not as deep as with shrubs, trees, and grasses. The first 2-3” of soil dries out quicker than at a deeper depth, particularly if you have soil with a high clay content.

The following factors may decrease the need for watering:

• A 2-4” layer of mulch surrounding the plant. (Keep mulch 3” away from trunk of plant)
• Transplanting a shrub or tree into an area that collects runoff water. 
• Presence of a hard, compacted soil layer that obstructs proper soil drainage
• Presence of shade
• Transplanting into a heavy, “clay-type’ soil
• Cool temperatures
• Abundant rainfall

Factors that can increase the need for additional watering: 

• Windy and/or sunny climatic conditions
• Plants installed on a slope
• Little or no rainfall
• Sandy soil
• Presence of large trees or shrubs nearby
• Abnormally high temperatures
• Transplanting the tree or shrub close to “hot’ surfaces (such as brick, concrete or asphalt)
• Plants that were grown in a very coarse soil mix (water drains through rather than being retained)

As the plants mature, Mother Nature may provide enough water to support them, but with new plants, monitoring the site/rainfall will help determine watering needs throughout maturity.  When creating a planting plan we encourage you to select plants that are drought tolerant when mature and plants suited to the installation site.  This helps conserve our precious water and reduces costs and maintenance time for you.