Air layering: turning one good branch into a second tree

Air layering is the cheapest way to get a mature-looking trunk, because the layer starts with the age of the parent. On a juniper it is close to foolproof; the failures almost always come from cutting the ring too shallowly or opening the bundle too early to look.

Why the ring has to go to the wood

Air layering works by interrupting the flow of sugars down the branch. Those sugars accumulate above the cut, and that accumulation is what triggers root formation. If any strip of cambium remains bridging the cut, the sugars continue downward and no roots form — the branch simply heals over and the layer fails, often without any visible sign until the bundle is opened months later and found to contain callus and no roots. The ring must be cut with a single clean pass on each side, peeled away in one piece, and the exposed wood scraped with the back of the blade to remove the thin film of cambium that clings to it.

Sphagnum, moisture and the right amount of air

The rooting medium has to be wet enough to keep the cut alive and open enough to let the new roots breathe. Soaked sphagnum squeezed out until it is damp rather than dripping is right. Too wet and the cut rots within three weeks; too dry and the cambium dies back. The plastic wrap is tied at both ends but not sealed airtight along its length, so some exchange happens. In hot weather the bundle is shaded with foil, which also keeps the moss from cooking. On juniper, roots appear in about eight weeks in warm conditions and fourteen in a cool season.

Reading readiness without opening the bundle

Through clear plastic the new roots are visible from about week six. They emerge white and translucent, and they are not ready. Over the next three to six weeks they thicken and turn a light brown as they lignify, and that colour change is the signal. Cutting a layer whose roots are still white usually kills it, because the roots cannot yet take up enough water to support the foliage they are suddenly feeding. Roots that have turned light brown and are visible for 3 to 4 cm around the ring are ready to separate.

Separation and the first month

The branch is cut below the bundle with a fine saw, in one pass, and the plastic is unwrapped. The new tree goes into a pot one size larger than the root mass, into the same mix the parent is growing in, and is tied in securely. It spends four weeks in bright shade with no direct sun, and is misted twice a day. No fertiliser for six weeks. On juniper about 80 percent of layers make the transition cleanly; the failures are almost always the result of exposing a layer to full sun in its first week.