What’s a rootstock anyway?
I don’t want to disturb you, but gardens all over the country are filled with trees that would make Frankenstein (the doctor, not the monster!) proud. These freakish plants are stitched together from the beheaded carcass of one tree and the dismembered limb of another, often from completely different species. And who’s producing these monster plants? Fruit nurseries, that’s who!
Silliness aside, almost no fruit trees are grown via sowing seeds or simple vegetative propogation nowadays. Instead, standardised ‘rootstocks’ are grown, then the top is cut off close to the soil and either a bud or a small cutting is attached. The new wood is from the variety whose fruit is actually wanted, and of successful the two genetically different bits merge. The rootstock handles everything below ground, and the ‘scion’ is the bit above ground you actually see.
Why do it this way? There are three main reasons:
- Many varieties of fruit tree are hard to propogate from simple cuttings and don’t come true from seed
- Rootstocks can be used to make the tree more or less vigorous than it might be on its own roots
- Rootstocks can often cause the tree to fruit earlier than it would otherwise
The most interesting thing about rootstocks is that often they are not even the same species as the scion. Apples are normally grafted on to very closely related apple species or hybrids, but pears may be grafted onto quince as well as pear, and medlars are basically never grafted on to other medlars but can be put on pear, quince or hawthorn roots. Plums tend to be on plum rootstocks, but those same plum rootstocks might also be used on apricots and other members of the prunus family.
Unfortunately, while there are quite a few standard rootstocks, very few are commonly sold so you normally only get 2 – 3 options per species/fruit, and those 3 are the same 3 everywhere. Here are the ones commonly used in the UK for the most common garden tree fruits:[table “3” not found /]
And here’s a chart from Wikipedia showing the effect on size for apples:
Keepers, who I bought my most recent trees from, offers a bit more variety, but even they mostly stick to the common rootstocks. That’s why I was excited to spot a couple of new and quite rare options on their website, which I’ll describe below.
The first surprising rootstock they are offering is ‘Mespilus’ for medlars. Since the only species in the entire genus is the medlar itself, this really means using medlar rootstock. This is not offered by any other nursery I know of, and as late as 2011 Hamid Habibi of the same Keeper’s nursery regarded it as problematic:
I don’t know about Amelanchier and would also be interested if anyone has used it. Rootstocks for medlars is a problem. Traditionally Crataegus has been used but has the problem that you continue to get large amounts of growth from the rootstock even when the trees are fully mature. Quince rootstocks are better in that respect. I would be very interested in trying seedling medlar as rootstock but medlar seeds are very difficult to germinate. Would be interested to hear if anyone has had any success in germinating medlar seeds and how
In line with the fact it’s new, Keepers offer no advice about its eventual size, or anything else really.
Own-root Apple Trees
The other surprise rootstock is more a lack of rootstock. Keepers are now also selling a limited number of ‘own root’ trees, all as 2 year old bushes.
I first read about own root apple trees at Cool Temperate. The long list of claimed benefits include:
- Better health
- Better fruit set
- Better fruit quality
The health claims are believable since most common rootstocks now are dwarfing, and they basically work by limiting nutrients provided to the scion and/or due to a slight incompatibility been rootstock and scion. It’s not surprising if that also renders the tree less robust in the face of pests and disease. In addition, because there are a few rootstocks, genetic diversity in orchards and gardens below ground is usually lower than above, which also makes the trees more vulnerable to disease.
Again, the big question is tree size. Keepers offers no advice, but Cool Temperate does. Here it is:
The only disadvantage of OR trees is that some varieties may be more vigorous than is usually wanted, though the average size of an OR tree is not large. This can be seen when observing self set OR trees grown from seed along roadsides, railway embankments, footpaths etc.. These trees are often quite small, and even when large seldom exceed the height of accompanying hawthorns.
The typical size of spur-type OR tree is similar to a tree on m9, i.e. 2-3 metres tall;
non-spur types are typically between M26 and MM106 in vigour;
and a few vigorous triploids giving trees between MM106 and MM111.
Size of tree is best controlled, if necessary, by early and regular cropping, which diverts the trees energy.
Another interesting question is how these trees are produced. I know that plums such as Yellow Pershore used to be propogated by suckers, but the main method I’ve read about for own root apples is to graft the variety as usual, then bury the graft union. If the bottom of the scion is exposed to soil it will often root, and voila!, job done. If this takes more time, it might explain why Keepers are only offering 2 year old trees. There is more about this here and here.
Cool Temperate has never, to my knowledge, actually sold any own root apple trees, despite promises that they were on the way. So it’s good that someone else has actually done it commercially. The only issue is that, probably due to the manner of production, only bush trained trees are available.