These are mainly rhizomatous, rexes, and tuberous begonias. With nearly all begonias you can root a leaf, but only certain types will then send up a new plant from the rooted leaf. With begonias other than the three types mentioned, consult with other growers about specific plants that may start from a leaf. Exceptions to the only rhizomatous and tuberous starting from leaves rule, are begonias such as B. luxurians and some of the mallet type canes.
Types of leaf cuttings
All parts of the leaf are capable of rooting and forming a new plant. The only requirement is that the leaf portion contain a main vein. There are three main types of leaf cuttings. A full leaf cutting, wedge cuttings, and cone cuttings. If your purpose is to create a number of plants, you may choose to do wedge cuttings since you can make many wedges from a single leaf. If your purpose is just to propagate a couple of new plants for yourself, you may choose to just use whole leaf cuttings. Cone cuttings are slower than regular whole leaf cuttings but since more veins are exposed to the rooting medium, the resulting plant is much fuller.
There are several basic requirements needed for starting leaf cuttings. You need warmth, good light, humidity, and a sterile moist medium.
Light and warmth
This is best provided by using fluorescent lights. A light stand, besides providing constant good light, also provides suitable warmth. Any area you can keep reasonably warm will work however. If not using lights, you need an area with bright light but no sun. Since leaves need to be rooted in covered containers, any sun will overheat and cook the cuttings. Under lights, you can keep the lights as close to the top of the container as possible. Leave the lights on for at least 14 hours a day. You can leave them on continuously if desired.
Most leaf cuttings need covered containers to root in. The purpose is to keep the humidity high and also to keep the medium sterile. The container can be as simple as a clear plastic cup covered with saran wrap for single cuttings or an expensive tray with a clear dome. You can even root leaf cuttings in zip lock bags. If you’re lucky enough to have a greenhouse, you can root leaf cuttings out in the open under a misting system. Even in a greenhouse though, you may choose to use covered containers for ease of use.
I know several growers that root in zip lock bags with individual bags for each cutting. One grower I know stapled the bags to the wall in out of way places during warm weather. For especially rare or hard to grow varieties, I usually do provide those cuttings their own container. I put the rooting medium in a small pot then put the pot into a zip lock bag after the cutting is in it.
Trays with domes or clear sweater boxes work very well. You can even use aquariums left over from your fish experiments. There are also a myriad of different clear sandwich or food containers to choose from.
You can either use the medium directly in the tray or use individual pots of medium for each cutting then set in the tray. Both have advantages and disadvantages. Just filling the tray with medium is easier and can be refilled over and over. However, in my experience if you root this way you end up with parts of the tray and different varieties of begonias growing at different rates. You usually end up with half of the tray potted up already and the rest still waiting. If you propagate continuous and keep refilling the tray as you take things out, it will work fine though. Another disadvantage is getting or keeping the medium to the correct dampness without being too wet. It’s also hard to keep the cuttings separated by variety as they grow unless you’re careful to make clear separations and labeling.
Using individual small pots for each cutting works well because you can move cuttings from box to box as needed. If you’re using several boxes as things get potted up, you can recombine the slower rooting cuttings into one box. The disadvantage is that it is more time consuming filling all the individual pots and making separate labels for each. If you don’t mind the added time, it’s the better method though.
Rooting mediums for leaf cuttings
The most commonly used medium for leaf cuttings is perlite. It is already sterile and holds the correct moisture without staying too wet. It’s only disadvantage is you have to check often to make sure it doesn’t dry out. Any medium such as peat moss, vermiculite and combos will work fine as long as they are made or kept sterile and kept to the right degree of moisture. I have used all the various mediums with good success but find perlite the easiest and best to use.
Another less common medium for rooting cuttings is called Oasis(TM). This is similar to the Oasis used for floral arranging but comes in form specifically for rooting cuttings in. It is made a size to fit the most common tray size. It has individual one inch cubes with a hole in the center of each to insert the cutting in. The Oasis can be cut easily to fit any container though. Don’t try using the floral Oasis for this purpose, it isn’t made for rooting cuttings like this other product is. The oasis is soaked in water till it has soaked up as much water as it can, then drained. It already contains fertilizer so nothing needs to be added to the water. It’s already sterile so also doesn’t need to have anything extra done to sterilize it. I have used it successfully many times and it works especially well for wedge and small cuttings. It was designed so that after the cuttings are rooted you cut the cubes apart and plant the cube and all in your potting mix. This design however doesn’t work for begonias. If the cube is left on the cutting the plant will usually not thrive or die later. The cube either wicks water to the surface of the mix so causes a dry spot, or stays too wet and causes the plant to rot later. Examination of the roots on plants that failed showed that the roots all stayed in the oasis instead of growing out of it into the mix. For this reason, you must remove all the Oasis from the rooted cutting before potting them up. This usually results in some root loss, besides being time consuming. However it is easy to use so does have its uses for some growers.
Other items you’ll need
One item you’ll need is something to cut the leaves with. You can use a knife, scissors, or pruners. The best cutting tool to use is a razor blade. There are several reasons. Using a new blade means you have a sterile utensil that doesn’t have diseases from your plants outside. If you use your pruners, you’d have to sterilize them. The main reason though is because it makes a very clean precise cut. If you use scissors or pruners they don’t cut cleanly and crush the edges of the cutting. This makes the cutting less able to draw up water. Using the razor blade cuts cleanly without crushing cells along the edge.
You’ll also need something to sterilize the cuttings with. It doesn’t matter how sterile your medium is if the cuttings you put into have spores of disease on the leaf surface. The most common disinfectant for using on cuttings is a five percent bleach solution. I have also heard of using a peroxide solution but haven’t personally tried that. I have also sterilized cuttings by dipping them in a fungicide mixed to the recommended strength on the bottle. I let them dry, then rinse with water before using. Make sure to wear gloves. You can also use Physan(TM) following the directions on the bottle. I usually spray my tray of cuttings with a fungicide after they are done just to make sure nothing was missed.
Whole Leaf Cuttings
A whole leaf cutting consists of a leaf with a portion of the leaf petiole (a petiole is the stem-like structure that holds a leaf to the plant stem). You should leave the petiole about one half to one inch long for rooting. When taking the cuttings leave the petiole long until just before you’re ready to put it in the medium so that the cut is fresh. Leaving the petiole too long won’t hurt anything. However, it will take longer for the plantlets to come up after rooting since they’ll have to come up from deeper in the medium.
In the illustration below you can see a whole leaf. The best leaf cuttings are young leaves but any leaf will work such as damaged leaves you have to remove anyway. If the leaf is small you can just cut the petiole and insert it into the rooting medium. Larger or damaged leaves you should cut down as in the illustration leaving a round center of the leaf with the petiole. The remaining part of the leaf can be discarded or used for wedges. The reasons for cutting the leaf down is that it takes up less space in the tray and because the petiole will have less leaf to support. The cut down leaf will have less leaf surface to transpire from so the petiole won’t have to provide so much water. Even if making wedges or cone cuttings, save that middle portion as an extra cutting. On difficult varieties, that portion will usually root, even if your wedges fail.
Whole leaf cuttings can be started without enclosed containers for some of the sturdier varieties. You can leave the petiole slightly longer and root them in small jars of water. You can also fill the small jar with perlite and add water. The second method does support the leaf better. You can also use pots of perlite set in a shallow tray of water. If you use any of these methods, don’t cover the container since the cuttings will usually rot with all that water if covered. It does take practice and experience to find out which varieties of begonias will work with which methods.
Credits / references:
Brad Thompson | Brad’s Begonia World