Azaleas can be propagated sexually, from seed, or asexually (vegetatively) from cuttings, layers, grafts or by tissue culture. Different seedlings from a cross between two different azaleas may exhibit characteristics of either parent and anything in between. While seedlings from a self-pollinated species azalea will tend to resemble its parent, even they will exhibit some variability. The only way to faithfully reproduce a particular plant is to make a copy of it (a clone) by vegetative means.
Think of layers as cuttings which remain attached to the parent until roots have formed. Layers occur naturally in the garden when low growing plants are heavily mulched, thus partially burying some of the branches. They occur naturally in the woods when tree limbs fall and pin branches of native azaleas to the ground.
To make a layer, find a branch that can be bent to the ground, dig a shallow trench an inch or two deep in the direction of the branch, and bend the branch down to put it into the trench. Cover at least a few inches of the branch in the trench with good soil, and hold the branch firmly in the trench with wooden pegs, U-shaped wires, or heavy things such as bricks or stones. It can help to wound part of the branch that will be underground by making a shallow cut or by scraping off some of the bark, and putting rooting hormone on the wound. Some people carefully twist and bend the branch upright at the point it is buried, tearing it a little as it is twisted, to form a sideways ‘T’ with the right hand side of the T pointing down. Although this works if the branch stays connected, the damage gives more opportunity for disease to enter the wound. The length of the branch beyond the point of burial is not important, and can vary from a few inches to a foot or more.
In about a year, the branch will have grown roots. It can then be cut loose from the parent plant and potted up or replanted elsewhere. Depending on the size of the branch and the amount of roots, it may need special attention to watering until it becomes established.
Mound layering can propagate larger quantities of hard-to-root plants. In the spring, cut off the existing shoots about six inches above the ground, which will force the plant to put out additional shoots. In the fall, mound a well-draining soil mix (any soil/sand/peat/leaf mold mixture) around the plant about four or five inches above the old ground level. It can help to build a box of wide 1″ boards to keep the mound in place. Keep the mounded soil moist for almost a year and a half.
Remove the mounded soil carefully the second spring after adding it, to expose the bases of the now-rooted shoots. Cut the shoots off just above the old ground level, and pot them up or plant them in a nursery bed and care for them like any other young plants. The mother plant will now grow a new crop of shoots, even more than before, and the mounding may be repeated in the fall.
Air layering is another way to grow roots on a branch if it cannot be conveniently bent to the ground. In early summer, when the new growth has begun to firm up, choose a branch. Make a shallow cut a foot or so from the end, in year old wood, parallel to the branch and about an inch long,and sprinkle it with rooting hormone. Then wrap a handful of moist sphagnum moss around the cut, and wrap the moss in a piece of polyethylene plastic to form a water tight enclosure. Use electrician’s tape or wire to fasten the plastic tightly to the branch. Check it monthly to be sure the moss remains moist.
An alternative to making a shallow cut is to carefully cut or scrape the bark from the branch to form a half inch or so band of bare wood. The rationale is that removing the bark will prevent nutrients manufactured by the leaves from getting back to the plant, and go into root growth instead.
In late autumn, carefully remove the plastic, and you should find the sphagnum moss full of very tender roots. Cut the branch from the plant, keep the roots moist, and plant it in a cool greenhouse or a cold frame where it will be protected from freezing until the roots have grown into the pot or bed.
Pot layering is a variation on air layering, in which you plant the branch in a pot while still attached to the parent plant. To do that, wound the branch, slit a one gallon plastic pot to be able to slip it over the branch, figure out a fairly permanent support for the pot, and fill it with a porous medium as for layering. Water the pot frequently to keep the soil moist. In about a year, the branch will have grown roots, at which point it can be cut loose from the parent plant.
Cuttings of the stems of most evergreen azaleas can be rooted rather easily. Use new growth for the cuttings, at any time from June onwards, depending on the weather and the variety of azalea. The exact timing depends on the condition of the cutting wood, which should be somewhere between soft and brittle: it should not bend like rubber, and it should not snap like a matchstick. At the proper time, it will probably be putting out a new terminal bud.
Take cuttings from the short shoots that come from the ends of existing wood, not the strong thick shoots that come from the base of the plant. Cut them two to five inches long, preferably in the morning, and preferably from a well-watered plant in good condition.
Cut off all but the top cluster of leaves, or carefully strip them off without also removing the tender bark. If the terminal leaves are quite large, consider cutting them in half. If the cuttings will not be stuck immediately, moisten them, shake off the excess water, put them in a plastic bag and put the bag in a refrigerator. Some people think overnight refrigeration is preferable to sticking them immediately.
When you are ready to stick the cuttings, wound the bottom half inch or so of each cutting by scraping off the bark with a knife or thumbnail. You can optionally use a rooting hormone. With a liquid hormone such as Wood’s, use a 5% solution (mix 1 part of the hormone with 20 parts of water), and put the bottom inch of the cutting in the liquid for no more than 5 seconds. If the solution is stronger or the immersion is longer, you may burn the cutting and it will not root. A talc-based rooting powder such as Hormodin or Rootone is safer to use. Put the bottom inch of the cutting in the powder and shake off the excess. In general, evergreen azaleas root well without any hormone.
The most commonly used rooting medium is 50/50 peat and perlite, although various other mixtures of peat, coarse perlite, sand, fine pine bark and vermiculite are used. The goal is to provide both moisture and oxygen. The medium should be from four to six inches deep, and may be in a flat for large quantities, or a one gallon pot for up to a dozen or so cuttings. Prepare it a few days before you will use it, and water it several times to moisten it well. Leave the medium loose to promote drainage and aeration.
If planting in a flat, use a straight-edge to prepare each row. Lay the board on top of the medium and use a knife to cut a two- to three-inch deep line along the edge of the board. Insert the cuttings into the cut every two to four inches, depending on the size of the cuttings and how long you intend to leave them in the flat. Move the board, cut a new line, insert more cuttings, and so on. In a pot, use a large nail or a pencil to make a hole for each cutting. Use a consistent convention for labeling the cuttings, such as a pot label at the beginning of each row, or a pot label at the beginning of the cuttings for each different kind of azalea.
After sticking the cuttings into the medium, water them in to settle the medium around the cuttings, preferably without wetting the leaves. Then cover the flat or pot with plastic to seal in the moisture. A large zip-lock bag works well with a one gallon pot, and a plastic tent can be made for a flat. Finally, put the flat or pot where it will get as much light as possible, with no direct sunlight. While brief exposure to early morning or late afternoon sun is not harmful, strong daylight sun will heat the enclosure and burn up the cuttings.
The cuttings should root in four to eight weeks. Be patient. While a very gentle tug can indicate progress, it can also break off any initial roots before they get established. When the cuttings have rooted, the enclosure should be gradually opened a little to acclimate the cuttings, and opened entirely in a few more days. Finally, transplant the rooted cuttings to pots or flats of peat moss, sand and leaf mold and keep them from freezing temperatures in a greenhouse or cold frame for at least one year.
Cuttings of deciduous azaleas are more difficult to root. The first problem is rooting them at all, and the second problem is getting them to break into new growth after they have grown some roots. The general procedure is the same as for evergreen azaleas. The differences start with the cuttings, which should be taken earlier, while the wood is still quite soft and green, usually around late May. They will require the use of a rooting hormone. While a stronger rooting hormone will increase their chances of rooting, it makes it harder to get them to break dormancy and go into active growth.
The cuttings will do best in a greenhouse with bottom heat from electric heat cables at around 75 degrees F, and with a mist system during the day to ensure they get all the moisture they can use. However, they can also be rooted under plastic as described for evergreen azaleas.
As soon as they have rooted, the cuttings should be fertilized with half strength liquid fertilizer and given three to four hours of extra light through the summer to force them into active growth. Use 75 watt incandescent bulbs or a mix of incandescent and fluorescent bulbs, as close as possible to the cuttings without burning them, which is about 6 inches away. In late September, stop the extra light to let the cuttings harden off for the winter. Do not disturb their roots until they begin to grow the following spring, at which time they may be potted up or moved to a bed in the garden.
Cuttings from azaleas which are difficult to root can be grafted onto an azalea with a good root system. The scion or cutting wood should be dormant, and the root stock in active growth. They should both be decidous or both evergreen. Using a greenhouse is best, to force the root stock into active growth in later winter or early spring, while the plant to be reproduced is still dormant.
Follow any standard grafting procedure, being careful to match at least part of the cambium layers. It is helpful to then wrap the union in damp sphagnum moss and cover the entire plant, scion and all, in a plastic bag. In about a month, after the scion begins to grow, gradually admit air into the bag until the scion hardens, after which the bag can be removed.
Azaleas grow readily from seed. The seed pods are plainly visible soon after the flowers drop, and become larger over the summer, eventually turning dark brown, splitting open, and dropping the seed on the ground. Only very rarely will the conditions be right for the seed to germinate and grow in the garden. Instead, collect the seed pods before they open, and grow the seed under controlled conditions. Collect the seed pods as they begin to turn brown, usually around the time of the first frost in the fall.
Put the pods of one variety into one container, such as an envelope or a small paper muffin cup, and mark it with the variety. After a few weeks, the pods will split and begin to spill out their seed, as many as 500 seeds per pod. Clean the seed by separating it from the pod and other debris.
Prepare some flats, which may be any size and material, by putting a mixture of sand and peat or leaf mold or perlite to within about an inch of the top, and covering it with a half inch or so of milled peat moss. Soak it until it is thoroughly wet, and let it drain out the excess water. It may be convenient to soak it separately in a tub of water, and fill the flat with handfuls of the mixture squeezed free of excess water. The plastic containers used by supermarkets for pastry and salads, about three inches high and various widths and lengths, are ideal for small quantities of seedlings.
Sow the seed in the winter, indoors, to give the seedlings as much time as possible before being subjected to outdoor winter temperatures. Broadcast it over the flat and mist it lightly to settle the seeds. Cover the flat tightly with plastic, and put it under artificial lights or on a north window sill. The seed will germinate in two to six weeks, depending on the variety.
When the seedlings have developed two sets of leaves, carefully transplant them into other flats filled with a similar mixture of sand and peat or leaf mold or perlite. Use a toothpick or similar fine instrument to remove a seedling, and to plant it into the new flat. Use a two or three inch spacing, as they will probably stay in this flat for a year or more. When the flat is filled, water the seedlings with a fine spray to settle the soil around the roots. Cover it with plastic, and place it under lights or on a north facing window sill, or outdoors in the shade if the weather is reliably above freezing. After a few days, the seedlings should be established and the plastic can be removed. Fertilize with very weak solutions of liquid fertilizer to maintain active growth.
After they have grown a year, the seedlings are ready to be potted up or planted out. The soil should be almost a solid mass of fine roots, and the soil can be cut into squares rather than trying to find out which roots go with with plant. They are quite tender, and should be given ample water and ample shade.
A good source of clean seed is the ASA Seed Exchange. The seed list is posted in January and members can order earlier than non-members. Collecting your own seed is fun, but if you want to diversify your collection try this source of seed.
There is a great deal of ongoing experimentation in propagating azaleas, especially with the harder-to-root and slower-to-grow-from-seed deciduousazaleas. Click here to read about several new approaches.