Propagating your own plants is one of the great attractions of greenhouse gardening and there are few things quite so satisfying as increasing your stock with beautiful home-grown specimens – for free. However, the essentially artificial nature of the greenhouse environment, coupled with the conditions required to encourage the new young plants to grow can sometimes bring their own problems when it comes to propagation. Although it is a rare gardener who does not run into trouble occasionally, a few simple steps can help avoid the worst ravages of many of the more common problems.
Prevention is Better Than Cure
Using only the strongest, healthiest plants as your source is the first trick in minimising propagation difficulties. By picking good parent stock not only do you stand the best chance of getting new plants of a similar high quality, but also avoid the likelihood of introducing disease or pests into the greenhouse. While no one would take cuttings, for example, from obviously diseased plants, some problems can be quite hard to spot – particularly some viruses and certain kinds of pests, such as eelworms – so it pays to be very careful. Most of the eelworms which affect greenhouse plants attack the leaves, so for those which are particularly prone to these problems, such as phlox, it is probably best to raise them from seeds or root cuttings, to avoid the risk altogether.
Following a good regime of greenhouse cleaning is another essential link in reducing the chance of problems during propagation – some pest, such as red spider mites, over-winter in nooks and crannies and will wreak havoc among tender plants if they once get a grip. Dirty pots, un-sterilised compost and poorly washed tools can all also introduce disease into the greenhouse, so a thorough clean up ahead of the growing season is essential to help keep things under control.
Many of the viruses which infect greenhouse plants can be easily passed to young seedlings or cutting, either from other infected plants, or via sap-sucking insects, such as aphids. Affected plants may show signs of discolouration, spots or streaks on their leaves, or suffer stunted growth. Ruthlessly destroying them is the only course of action and their remains need to be burnt or discarded well away from anywhere where they might infect anything else – definitely not on the compost heap! Any aphids present will also need to be eradicated in the usual way. Not only are they serious vectors of disease, but their sugary excretions also encourage moulds to grow, which is certainly something to be avoided.
The frequent watering and high humidity which are so often a feature of greenhouse propagation can also provide ideal conditions for a range of moulds and fungi to thrive, including the likes of Botrytis and various mildews. “Damping off” is a particular problem for young seedlings. Spreading rapidly in wet compost, particularly among densely sown seeds, the fungus attacks the base of the stem, making it soft and the plant flops over and eventually dies. Proper hygiene when sowing can go a long way towards helping to prevent the problem, as will ensuring that the seed trays are not too over-crowded and providing good ventilation.
Blackleg is another fungus likely to be a nuisance – this time affecting cuttings. Often brought into the greenhouse on dirty containers, it usually occurs just as the roots are beginning to form. Commonly the first sign of infection is a darkening of the stem at the bottom of the cutting, which then shrivels, being followed a little later by the discolouration of the rest of the plant and, finally, death. Fortunately, the risk of blackleg can largely be avoided if you use rooting powder which contains a fungicide in the mix and clean all pots thoroughly. It can also be a good idea to use only mains water, since if the fungal spores get into your water butt, they can easily be spread from plant to plant.
For anyone interested in bringing on their own plants, especially in cooler climates, a greenhouse offers a huge advantage. Like any other form of gardening, there are never any automatic guarantees of success and sooner or later disappointments are almost inevitable, but with a little care and a few simple precautions, some of the more common problems can often be avoided.