Aspen grafting

Aspen (Populus tremula) trees in Scotland rarely flower or set seed, nor do they readily grow from stem cuttings, so Tree for Life’s aspen propagation work has focused on a method using root cuttings. 

As aspen naturally spread by sending new shoots, called ‘suckers’, up from their root system, we can encourage this suckering by bringing root cuttings into a polytunnel in early spring, and then harvest the shoots as they come up, rooting each cutting individually using a mist propagation unit. This method, despite a good success rate of usually between 80 and 95%, is very labour intensive; hence trees are expensive and not available in such large quantities as other species that are readily propagated from seed.

Another charity called Coille Alba, which leads the Aspen 2020 Project, one of whose main objectives is to increase the area of aspen woodland in Scotland, has been looking into methods of stimulating aspen to flower and produce seed inside a polytunnel and also working on creating a ‘seed orchard’ for aspen. Working in conjunction with Coille Alba this year we have been producing grafted aspen trees destined for a future ‘seed orchard’ or ‘gene bank’. Cutting material has been collected from the top of mature aspen trees, which are known to be aspen clones more willing to flower than others – this material will not itself root readily, but it can be grafted on to the top of another young aspen tree. Abbey, our Tree Nursery Assistant who did most of the grafting work early this year, has taken small cuttings with about three buds and cut the end of the stem in a ‘V’ shape to carefully match the cut made into the side of the stem of the rootstock plant. The cutting is inserted into the stem of the rootstock and taped so that the cambium layers of the two stems are held tightly together, and waterproofed, until the plants unite. The shoots of the rootstock plant are carefully trimmed back to encourage the new shoot (scion) to grow instead. Despite some initial problems with removing the tape when the plants started growing, the grafting trial here has been a great success with 84% success rate, and the plants grew prolifically in the polytunnel, so much so that they had to be cut back before being moved outside for hardening up later in the year. 

The grafting process is known to encourage prolific and early flowering in other species, and there has been some success with producing aspen seed from Coille Alba’s early trials, and this looks like it will increase year on year, so we plan to continue working together on this project in the coming season. If seed can be obtained from such orchards, it is hoped that trees propagated from them will be abundant and inexpensive, thereby dramatically increasing the availability of young aspen for native woodland planting schemes. 

Jill Hodge

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