Top Tips for Rooting Stem Cuttings
It’s July, and an ideal time to take stem cuttings so the new plants can develop a root system before the winter. There are many articles and videos on this topic, and most will suggest covering the pots with a plastic bag or coke bottle to maintain a high moisture level to prevent the cuttings from drying out. While this works, the pots still have to be regularly watered and a framework of sticks may be needed to prevent the leaves making contact with the sides of the bag and rotting.
The method described in this article is much easier, requires very little maintenance and uses plastic storage boxes to maintain high humidity levels for fast and reliable rooting.
Taking Cuttings
Select the shrubs you want to use and take cuttings from non-flowering stems if possible. Here’s the science: Although the mechanism of flower inhibition of rooting is not known, several possibilities exist. First, a metabolic change may take place in the leaves following flower bud initiation which results in a reduced production of "cofactors" necessary for root growth. Second, a system directly inhibitory to rooting may follow flower initiation. Third, flower buds may be better competitors for something made in the leaves that is required by both roots and flowers. These mechanisms are probably further complicated by plant age and/or maturity of the cutting material.
However, the lacecap and paniculata plants in the images are so floriferous that there are few non-flowering stems to select, so the flowers are simply cut off. This produces plants branching at soil level, rather than a single stem from a non-flowering cutting, (which later branches), as can be seen in the image of weigelia ‘Apple Blossom’ and the hydrangeas. They appear to root just as well, and if I want to take cuttings from certain shrubs, I have no choice.
Here are cuttings of hydrangea, philadelphus, hypericum and fuchsia Hawkshead before and after trimming. Remove the bottom leaves, leaving a pair, or two pairs at the top. I’m usually quite aggressive with leaf removal as this means there can be more cuttings per pot, but if you are taking only a few cuttings, more leaves can be kept, or not cut in half, and spaced more widely in the pot. Some stems, such as philadelphus, can have weak and floppy top growth, so cut that off as it will likely die back. The cutting should be semi-ripe, that’s this year’s growth, firm, but not floppy. If the stems are not firm enough, wait a few weeks until they harden. Woody stems, which are last year’s growth, don’t take well, and are better rooted into open ground in the autumn, but the box method produces plants faster and more reliably.
The length of the cutting is not important, but make sure the pots are deep enough for several leaf nodes to be below soil level – there are higher levels of undifferentiated cells around the leaf nodes, and rooting tends to begin there. One-litre round pots are ideal as six fit into a 60x40cm box. The top of the cutting should be well below the top of the box. While taking cuttings from a parent plant, take a bucket of water or plastic bag to ensure the stems don’t dry out. Once the cuttings are prepared, dip the stems into rooting gel or powder, and then push into the growing medium. For softer stems, use a stick to make the hole, down to the bottom of the pot. Lacecap and mophead hydrangeas will root if you look twice at them, but rooting powder should help.
Water the pot well, and put into the box. The rooting medium should be well soaked, to the extent that a thin layer of water should be at the bottom of the box. While this is counter intuitive, and it might be thought that the cuttings need only a moist compost so that air can be present to encourage rooting, I have found that a high level of moisture is most effective, to the extent that it evaporates, condenses and runs down the side of the box – here we are replicating the misting system that commercial growers use. If you have a shallow mesh tray, try placing the pots on that, but I have found it’s not necessary. This box method provides a very high level of humidity, which is the key to its success. The box should not have holes and the lid should be kept on. Place the box below the greenhouse bench, or in a bright position, but not in direct sun. The heat in the greenhouse will encourage faster rooting, and the cuttings don’t seem to mind the higher temperatures. If the box is placed in a cool location, rooting will take longer. I have used a heating mat below the box, which works well.
I use my own compost from 50% well composted shredded branches (not grass based garden compost) and 50% two-year-old leaf mold, which is very free draining. But try any good store-bought compost, perhaps trying a 50% mix of sharp sand (not builders’ sand) or perlite in some pots to see what works best for you, as store bought peat free composts are very variable in quality and how friable they are.
Which plants to root?
I have around a 95% rooting success rate for the ‘easy’ common garden shrubs, including roses, but have found that rhododendrons, azaleas and camelias don’t root so well with this method, so I’m trying air-layering this year. But give any plant a try as there’s nothing to be lost. Any stem rooting herbaceous plant such as geranium Macrorrhizum or Dalmaticum takes well and quickly with this method, and while the lovely Rozanne won’t root from the stem, pull the soil back and take some plump root divisions, pot on and put in the box.
Aftercare
In many respects, rooting the cuttings is the easy part, but they require early potting on, as the saturated rooting compost is not appropriate for continued root growth, which does require a free draining medium. Most of the cuttings should root in 3-6 weeks, depending upon the temperature inside the box – a constant temperature of 20C would be ideal, but unless a heated mat is used, that won’t happen. Check the pots every week and remove any fallen leaves which will rot if left. After a few weeks, remove a pot and check that the cuttings are developing top growth. Give the cutting a gentle tug to see if there is resistance from new roots, which will eventually appear at the bottom of the pot if left. If there are roots, gently tip the cuttings onto a tray and remove those that have rooted and pot on, replacing the unrooted stems back into the pot. Those unrooted should at least be developing callouses at the bottom of the stem, and some take longer than others.
Water the rooted and potted cuttings and place in another box and put the lid on, but make sure the compost is moist, but not saturated. The plants have become used to the warm, humid conditions, and may fail at this stage if left in the open. Allow them to develop in the new box for a week or so, leaving the lid slightly off to one side to acclimatize to life outside the box. Don’t place them in direct sunlight until they are clearly established and growing well. Small 9cm pots are a good size for the first potting on, and since it’s likely now late August or early September, there won’t be much root development or branching before the end of the season. If there is, pot on an 11cm pot. Place the pots in a protected position and don’t let them become saturated by autumn rainfall – a cold frame or greenhouse would be ideal, but do keep moist.
Winter Care
You are more likely to lose your new plants during winter than any other stage unless they are well cared for, and that means not too wet, and not frosted. By October, they will most likely be too small to plant out, so wait until the leaves fall, and put into a greenhouse, garage or shed. Beware of hard frosts – I lost around 30 young plants in a greenhouse when the temperature unexpectedly fell to -10C. Do not allow the pots to freeze.
In spring, bring the plants out of their winter home and water. For the plants in the images, 90% of the growth come in the spring, in my location, from March to July, with potting on from 9cm (0.75L) to 1 litre pots, then 2 or 3 litre.
A cold frame is ideal. They are far too small and costly to buy, so I made my own from Axiome Clear 10mm Twinwall Polycarbonate Sheet, bought in 3000mm x 1000mm lengths. It’s easy to screw through the sheets into timber supports and use T-hinges for the roof sections. That gave me 6 sqM of growing space for £300. It breaks down for garage storage come the winter and there’s no worry over glass breakage.
Our many thanks to Les, for sharing these wonderful tips. A wonderful read for money saving gardening this summer.
