Clonal propagation of pome fruits is the only method of generating additional trees of particular varieties, as these fruits don’t come true to seed. In other words if you plant a seed from that delicious apple or pear in your lunch today, after growing for 7-10 years, when it produces fruit in all likelihood the fruit will be nothing like the “mother” fruit that it came from. The resultant fruit could be bitter, sour or sweet and any combination of those components of taste. Therefore, if you want to grow your own Honeycrisp or Jonagold apple tree, or perhaps something more obscure and interesting such as Zabergau Reinette or Esopus Spitzenburg the tree will need to be produced by grafting a scion and rootstock together. Grafting is easy to learn and with an hour or two of practice you can be successful. However, this is not the place to discuss grafting in detail. For that have a look at a whip and tongue grafting article here.
As I mentioned above a scion and rootstock are required to clonally propagate pome fruits. The scion is simply the previous year’s growth, harvested and grafted while dormant. The rootstock is the part of the new tree that will go into the ground and provide the “stalk” if you will for the new tree.
There are a number of methods to propagate rootstock, a great overview of which can be found in Stella Otto’s book The Backyard Orchardist. The most popular method for commercial rootstock producers is what is called stooling. On the most basic level stooling is a simple way for anyone to propagate their own rootstock in their home garden or orchard.
Before moving on you need to be aware that some rootstocks are under patent. Many rootstocks are public domain, M9 or M27 for example. This means that if you buy an M9 rootstock you are free to do with it what you like, be it graft a scion to make a tree or plant it un-grafted to propagate your own rootstock. Some newer rootstocks however are under patent and do not allow anyone to propagate them unless a royalty is paid to the patent holder. This is true for the apple rootstocks created in Geneva, NY. You will see them in nursery catalogs with a G instead of an M as the rootstock code. Some of the more common ones are G16, G30, G41 and G65 and there are benefits to using them in your orchard but rootstock choice is an entirely separate topic. There are many non-patented apple, pear and quince rootstocks that you can use to experiment with stooling.
That settled we can move on to the process of stooling. The first step is to procure a rootstock of your choice that you would like to use for use in your orchard. Lets say you pick M9 as an example as this is a very common dwarfing apple rootstock. When it arrives you will need to find a nice spot to plant it. The soil should be well drained and loose. The dwarfing characteristic of M9 and other dwarfing rootstocks is achieved in part by having a smaller root system than seedling or standard stocks so be sure not to plant in hard clay or a really sandy site where it may struggle to grow or for water.
Set the rootstock into the planting hole on a small mound of soil and spread the roots out so they are spaced around the mound. This encourages them to grow down rather than out or up. Place the rootstock at a 45° angle. It may look funny but doing so will result in production of a greater number of rootstock per year simply by increasing the surface area of the plant. At this point all you need to do is water the plant and let it grow for one growing season.
After leaf fall, sometime during dormancy (December to early March in most years in the Pacific Northwest) you will do what might be a bit unnerving, get out your loppers and cut the rootstock down to or just below ground level. The root system remains in the ground and we can now call it the mother plant.
As the plant awakens from dormancy and begins to grow you will be delighted to see that a number of individual shoots emanate from the mother plant just below where you lopped off the stalk. The first year it may only be one or two, but in subsequent years it may be 5-10.
In addition to watering during the growing season you will need to mound up sawdust or another light and airy rooting medium around the shoots as they grow. Do this a few times during the growing season and be sure to keep this mound watered. During this time the shoots recognize the dark, damp rooting medium as being below ground and direct their latent buds to become roots and grow into the medium. This is why the mount must be kept wet, if it is too dry then the shoots will not send roots out to explore it.
During the next dormancy period you will reap the fruits of your labor (however minimal it was) when you gently brush away the sawdust from around the shoots (as in the picture above). Slowly the newly formed roots produced by the new shoots will be revealed to you. At this point each one of these shoots can survive on its own when removed from the mother plant. As you may surmise, the next step is to get out your hand pruners and cut each shoot at or below ground level, which should leave 4-8” of root area on the shoot. Each shoot that you remove is now an individual rootstock (an example of which is in the photo below) that you can graft a new scion onto. If as in the case of M9 you can then have fruit from this graft in 3-4 years.
Stooling is great in that it is a renewable process. The mother plant remains in the ground and so long as you take the time to mound up sawdust around the shoots and water the plant it will produce rootstock each year for 30 years or more. At that rate you will likely run out of space in your orchard for more trees before the mother plant stops producing new rootstock. But this gives you the opportunity to share your passion with friends and family by grafting their favorite varieties and giving them as gifts.