Cordons are a single main stem, with very short side branches which are pruned back hard to only 10cm or so from the main stem. Fruit appears on spurs on these short side shoots, and this method can be so intensive that the trunk is almost hidden from view by the fruit. They are pruned so tightly that they can be spaced only 1m (3 feet) apart, so you can get a lot of fruit in a very small space.
Formative pruning. You can buy 3 year old ‘ready made’ cordons from specialist nurseries, or else you can start with a one year old maiden tree. In either case, you will need to do some formative winter pruning for the first few years to extend the cordon to the length required. This is simply a case of pruning the main stem back by about one third of whatever growth it has made the previous summer. Prune to a nice healthy bud, anytime when the tree is dormant in a December or a January. This encourages the four or five buds beneath this cut to break and grow away the following spring. The top bud you will then train in as a new central leader - tie in to a bamboo cane to straighten it out - and the lower branches will become the productive side shoots where fruit will eventually form.
It is summer pruning, which removes vigour from the tree, which is when the main pruning will be done. The key to remember here is that we are trying to keep the tree in check and stop it growing away and out of our trained form. The reason cordons can be so prolific in producing fruit is that the fruit buds produced at the base of a branch are much more productive than those further up. This was first noticed by the French gardener and writer Louis Lorette in the late 1800’s and has formed the basis on which fruit tree pruning has been practiced ever since.
The ideal time to prune is around the middle of August. The tree will have done almost all its growing for the season by then, so you only have to do this once. You can prune earlier in the summer if you are bothered that the extension growths are making the cordon look untidy, but you will have to repeat the pruning again in late summer.
Summer pruning of cordons is very easy - simply prune back all side shoots to about 3 leaves. The precise length you leave on doesn’t overly matter, some people prefer to prune to 4 or 5 leaves from the main stem. The main point is to keep these lateral growths short, encouraging the production of basal fruiting spurs and also allowing in light and air to help fruit ripen properly.
Choice of varieties
Almost all of the pome fruit (apples, pears and quince) can be successfully grown as cordons. They respond well to the pruning regime and produce fruiting spurs with relative ease. You may see some apple varieties described as ‘tip bearers’ and worry that these will not produce fruit if trained as a cordon. Don’t fear - all apples have the capability of producing fruiting spurs, it’s just that some, if left to their own devices, will produce at the tips of their branches. However, cordon pruning forces them to produce spurs, as they are genetically programmed to reproduce and set fruit. The only variety that I have ever known which steadfastly refuses to produce spurs is the early eating apple ‘Irish Peach’, but otherwise the world is your oyster. Pears in particular are extremely suited to growing in this way - it shortens the otherwise long wait for trees to start fruiting and allows you to grow several varieties in a small space, improving pollination and fruit set.
The received wisdom is that none of the stone fruit (plums, damsons, cherries, etc) can be grown as cordons, and you won’t find many nurseries offering them pre-trained. This is touted as being because they set the majority of their fruit on one year old wood, rather than the permanent framework of old growth which is where apples and pears crop. However, there are several mentions of cordon-trained plums and gages in old text books from the 19th century, and some growers today report success in getting fruiting spurs to form. The main issue is one of disease - all members of the Prunus family are susceptible to diseases such as silverleaf and bacterial canker, and one of the main ways they enter a tree is through pruning cuts. The sheer quantity of such cuts necessary to keep a cordon in check dramatically increase the probability of disease taking hold. Pick one of the newer, disease-resistant varieties, seal cuts, disinfect your secateurs - maintain scrupulous hygiene and you have more than a fighting chance. If you are desperate to grow a plum tree and don’t have the room for any other shape, it would be well worth having a go.