Ask Mr. Smarty Plants

It sounds as if your friend's western red cedars are doing very well to have doubled their height in four years time.

Thuja plicata (western red cedar) is native to the northwestern United States, Canada and southwestern Alaska.

I am including references with more information about Thuja plicata from Washington State, The Gymnosperm Database, Glacier National Park and the Center for Wood Anatomy Research of the US Forest Service.

When you speak of taking cuttings, I am not sure whether you mean: 1. when and how to prune the trees for shaping, or 2. how to take cuttings for propagating the trees vegetatively.

Most sources say that T. plicata doesn't require pruning but will tolerate it if you want to make it a different shape or size.

There is lots of information about propagating T. plicata from seeds, but very little about vegetative propagation. The one source I found, Plants for a Future, recommends taking cuttings of half-ripe wood, 5-8cm with a heel in July/August and rooting in a shaded frame. Roots should form by the end of September but it should be overwintered in a frame. Alternatively, cuttings of almost ripe wood, 5-10cm with a heel, can be taken in September and rooted in a cold frame. Roots should form by the following summer and be planted out in autumn or spring. Here is a little more information about Plant Propagation by Stem Cuttings: Instructions for the Home Gardener.

With regards to the frost tolerance of the roots, Western red cedar survives in Ketchikan, Alaska with an average minumum temperature of 28.3 degrees F. in January, which is not too different from Bonn, Germany where the average minimum temperature in January is 31 degrees F. Unless you are exposing the roots completely, I would think that they could withstand the frost as long as they had at least a meter of soil covering them since I wouldn't think the soil would freeze down to 1 meter in Bonn's climate.