Is it normal for swamp cedars to get thinner in the winter? We have a row of swamp cedars at the back of our backyard that looks thinner than in the summer/fall (i.e. I can see through them when before I couldn’t) and I’m worried they are dying.
The native white cedar (Thuja occidentalis) is particularly prone to thinning in the lower branches when planted close together. The crowding reduces the sunlight reaching the lower branches, and often lose their foliage. If you’ve ever hiked through a cedar forest, the lower dead branches fool you into thinking an easy hike. However, the pointy dead branches that seem to interlace with one another quickly informs you that it’s not as easy as that!
You do not say that the cedar hedge had been sourced from an actual swamp, nor if the hedge backs onto a ravine or other natural or naturalized area. Cedars that are actually sourced from a swamp are often thinner than those that have been nursery grown. If growing adjacent to a natural backdrop, such as a forest or even an overgrown field, the trees, shrubs, and plants growing behind the cedar hedge will often provide the fill of green. Once the trees and shrubs lose their leaves, and the plants die down, this opens up the view, and the cedar may appear thinner.
If neither of the above is the case, then some proper maintenance might be called for. Pruning is essential to keep cedar hedges thick and healthy from top to bottom, otherwise, thinning foliage and dying branches from the bottom will happen within a few years of planting. Water also is essential, as cedar are naturally in moist conditions. Fertilization will help with new growth.
Within a year or so of planting, light pruning should be done when the new growth is visible either each spring, or at the very least, every other spring. It helps the new growth stay closer to the trunk, rather than reaching out to find the sun.
Deep watering also is essential. This means a soaking each week – to about 2.5 cm when you stick your finger in the soil to see that it is wet. Some people use trickle hoses to soak the base of the tree, rather than sprinklers, which gets everything wet. To maintain moisture longer, a good mulching around the base of the hedge will help. To a depth of about 10 cm, be sure the mulch does not touch the trunks. That can encourage disease.
Fertilizing is best done between the end of April and the end of June. This is the ideal period where the hedge will grow the most. Most evergreen tree and shrub fertilizers are high in nitrogen that encourages growth of foliage. The most common fertilizer for your cedar hedge is (18-4-8) and can be purchased at most garden centres or big box home improvement stores. On the growing branches, some lateral growth in the coming spring, may fill in the gaps you can see from your side of the hedge.
Over the next few years, with regular watering, fertilizing and proper shearing in early summer, the hedge should thicken and hide the bare areas to some degree.