Layering is a passive form of propagation. With certain varieties it happens all on its own. There are many different layering techniques, but they all follow the same fundamental concept: A stem is encouraged to form its own roots while it is still attached to the plant.
A branch is bent to the ground and weighed down with soil and stone until it roots. The stem is propped back upright with another stone. Rooting times vary depending on species and conditions, from months to years.
Simple Layering
Here a low branch is bent down to the ground. You can place a stone on top to hold it in place. For many species that are easy to root, this is enough. For more difficult species, you can tie a twist-tie wire around the stem. The wire will restrict growth as the stem expands through the growing season. The branch after the wire will feel the stress and be encouraged to throw down its own roots. You can also wound the stem by scraping it where it will be in contact with soil. This wounding will encourage callous tissue to form. Some growers dust the wound with rooting hormone.
Mound Layering
Instead of bending down a single branch, in mound layering you pile dirt or old sawdust over the whole bush. By the way, you can make a tree into a bush or encourage a bush to make more stems by cutting the whole thing to the ground in the winter.
In mound layering you should replenish the growing medium around the stems at least a couple of times throughout the season, as it will settle quite a bit.
Create a mound layer by simply adding a growing medium all around the stems. After rooting, pull the medium back and clip out the rooted stems for replanting.
Tip Layering
This is where just the tip of a stem is buried. Certain varieties naturally tip-root, and these are the ones this technique is best for. You can encourage more tip rooting than the plant would naturally make by weighing the tips down with stones. You can also allow grass to grow fairly thick around tip-layering plants. The grass will help the stems stay in place on windy days or when brushed by.
You can encourage a plant to make more stems for tip layering by pinching back the tips of stems in early summer. This will cause the stem to branch out, and each branch can create another rooted plant.
Some species that naturally tip-root are black raspberries, blackberries, gooseberries, and forsythia.
Serpentine Layering
This works best on vines and some very fast-growing shrubs. As a layered stem is rooted and continues to grow, the next section of growth is buried, and so on. I have had individual goji berry stems layered six to eight times in a season.
Air Layering
This is a very time-consuming technique, but is especially useful if you really want to have a certain tree on its own roots. It makes sense to have certain trees on their own roots and not grafted for lots of reasons, but especially if you want to build stool beds with them.
To air-layer, bring the soil up to a branch that is too high to bend to the ground. Wound the stem heavily; many growers peel off an entire section of bark. Take a plastic bread bag and cut the bottom out so that you have a sleeve of plastic. Slide it over the wounded stem. Tie the bottom tightly with a twist-tie. Fill the bag with moist growing medium. Many growers use peat moss, coconut fiber, or old sawdust. Fill the bag completely. Close the top with another twist-tie. Shade the bag by wrapping it in either aluminum foil or burlap. Check back every few weeks to see if roots are pushing against the sides of the bag. Once they are, clip out the stem and you will have a very large rooted plant.
Stool Layering
Stool beds are where plants are grown just to be layered. They are cut down to the ground every year as layers are harvested. It makes for a simple, effective propagation technique.
There are two methods I use for starting a stool bed. One is to plant trees horizontally. The second is to plant a tree and cut it down in the dormant season to turn it into a bush (this is the same as mound layering, but in this case you are building a whole bed). With either method, your goal is to have an entire bed sending up a dense stand of shoots.
When trees are planted horizontally, the trunk and branches are laid along the ground, just at or below the soil surface. All of their buds will grow skyward. As the stems grow over the summer, pile old sawdust or soil on them until just the tips are showing. I do this three or four times over the summer. The stems will root into the medium you pile on. You can think of this as similar to hilling potatoes. Just keep piling medium onto the stems, leaving the tips exposed. In the fall or the next spring, simply cut out the stem with some roots attached.
Planting a young tree horizontally causes many leaf buds to turn into vertical shoots. Each of these vertical shoots is mounded with sawdust. This is a fast way to start a stool bed. Illustration by John Walsh.
After you harvest the rooted layers off a stool bed, feed the soil with compost or some nutrient-rich mulch. As the years go on, stool beds develop amazing soils and excellent growth. The roots of the mother trees just get bigger and more mature, while the soil is constantly fed by incessant mulching.
Most commercial apple rootstocks are grown in stool-layered beds. Millions of trees are produced this way at a rate of around 60,000 rooted stems per acre.
Harvesting Layers
You can harvest rooted stems anytime. I like to let them get quite strong before I cut and dig them out. Sometimes I leave stems to root for two or three years before separating. The resulting plants can be huge.
Some plants take only a month or so to form roots, but others require two years, and still others may never do it. It’s fine if it takes a while. Layering requires almost no energy on your part once you’ve started it.
It is preferable to harvest layers while they are dormant plants. A pair of pruners or loppers and sometimes a shovel are all that’s needed.
All my stock plants for layering grow stronger every year and send up bigger and more numerous stems each season. They are fed generous helpings of compost and mulch. Layering is slower than using cuttings, but also much easier. Watering needs are near zero with all the mulch. I think layering is one of the most overlooked, easiest ways to propagate lots of plants. If you do it intensively, it can be quite productive and profitable.