By Scott Aker
Special to The Washington Post
Thursday, March 26, 2009
Q I planted some junipers 10 years ago that are out of control. They are now five feet tall. Can I prune them without killing them off?
A Junipers, like most conifers, do not have the ability to sprout new growth from the cut stubs of branches. Great care should be taken in pruning them so you don't end up with a disfigured shrub that is more a liability to your landscape than an asset.
The pruning protocol is simple. Identify the longest branches and follow them down to a point where they meet up with another branch, and cut off the long branch entirely. This will likely leave bare areas in your junipers. Repeat this pruning every few years to keep the juniper within bounds.
You may want to take more drastic measures and replant with another conifer that will fit the space more closely. One of my favorites is Silver Spreader, a short and spreading form of our native red cedar, Juniperus virginiana. Because it never grows taller than three feet, you can cut long branches back from the edge as described above and allow new branches to grow out to cover any bare areas created in the process. It is also less susceptible to pests and diseases than other species of juniper.
I have winter-hardy plants in containers on my roof deck: juniper, holly, boxwood and ivy. Should these plants be watered during the winter? I worry that freezing and thawing of water at their roots may harm them.
Shrubs and perennials in containers benefit greatly from watering when the weather turns mild during the winter, but make sure that most of the water has drained before freezing weather arrives. The soil in pots may not dry out rapidly in winter. Use your fingertip to check the moisture level. If the soil is moist a quarter- to a half-inch below the soil surface, no watering is needed.
If you can, move pots containing evergreens to a more sheltered location if cold, dry, windy weather is expected. With the soil frozen, the roots are unable to take up water, so giving them protection will slow water loss from the leaves and prevent or limit winter leaf scorch.
Scott Aker is a horticulturist at the U.S. National Arboretum.