The main cherry fruit tree rootstocks – Colt and Gisela 5 – produce 3 and 4.5m tall trees respectively. Gisela 5 is preferred in small gardens, while Colt is preferred in larger gardens where you want a free standing tree.
What are Rootstocks?
Rootstocks are plants, usually within the same genus, but not always, on which a variety is grafted. Trees are produced this way as trees grown on their own roots are simply too large for the average size garden. Rootstocks thus exert a dwarfing effect, producing smaller, more manageable trees. They also produce earlier in their life – an effect that is known as precociousness. This provides a huge benefit for growers, who can establish high density orchards from which they receive returns earlier.
History of Cherry Rootstocks
Rootstocks have been used for millennia and the earliest cherry rootstock we know of is Mazzard, which was used 2,400 years ago by ancient Greek and Roman horticulturists, and is still in use today. It’s excellent compatibility with cherry varieties and ease of management left it a favourite until the emergence of Mahaleb, which emerged in eighteenth century France. Mahaleb’s elusive dwarfing effect and increased precociousness made it the rootstock of choice until the discovery of compatibility issues in the 1920s led to the re-emergence of Mazzard.
It took till the second half of the 20th century for the classic rootstocks Colt and Gisela 5 to be introduced. Colt originates from East Malling Research Station, and was bred in 1958, but only released in 1977 after extensive testing. The rootstock produced a tree significantly smaller than Mazzard and with increased precociousness, and was importantly free standing. Also emerging at this time was Gisela 5 that originated from Justus Liebig University in Giessen, Germany in the 1960s. The rootstock had the greatest dwarfing effect of any rootstock hitherto and became the standard rootstock for small gardens and many growers. The only issue was Gisela 5 required staking.
Although a girl’s name in Germany, Gisela actually stands for Gliessen Selection for Prunus avium. Both Colt and Gisela are hybrids of species P. avium – the wild cherry – which were bred with the dwarfing Prunus species pseudocerasus and canescens respectively. The breeding programs wished for the compatibility of avium with the smaller size of the dwarfing species. The Colt program wasn’t entirely successful with the rootstock being incompatible with both Van and Sam varieties. Gisela, however has no issues as of today.
Colt vs Gisela 5
Note: standard refers to a tree grown on its own roots. Thus a rootstock that produces a tree 30% of standard will produce a tree 70% smaller than if it was grown on its own roots.
Colt produces a tree 3.5-4.5m tall (about 60-75% of standard). It does not require staking and will produce a free-standing tree.
Gisela 5 is roughly equivalent to the apple m26 rootstock, producing a tree 2.5-3m tall (about 40-50% of standard). It requires permanent staking.
Left unchecked, Gisela 5 is unsuitable for productive varieties such as Lapins and Sweetheart as it produces too many fruit buds, which it is unable to deliver sufficient sugars, leading to small, maldeveloped fruits. To address this, you can remove buds. Colt on the other hand is suitable for productive varieties.
It’s worth noting that rootstocks exert a dwarfing effect as they are worst at gathering and transferring resources than standard rootstocks. Gisela 5 is reported to perform poorly in shallow soils. This shouldn’t be much of a problem if you are careful to improve the soil, but if you are new to gardening Colt may be a better choice as it is more forgiving.
A problem with sweet cherries in general is that they take the longest time of any popular fruit tree to bear fruit. Gisela 5 will come into bearing in its third year, and Colt a year after, although it is important to note your tree will arrive 2 years old with a 4 year old rootstock. Imagine precociousness as the speed with which you get paid back on your investment.
Gisela 5 causes trees to flower earlier by a few days, leaving blossom susceptible to frost damage. This is a problem with cherries as they flower early in the year (March-April). This is important as flowers eventually turn into fruit. Another problem with Gisela 5 is that as it produces smaller trees, the branches are lower to the ground. The lower a branch, the more susceptible it is to frost damage as cold air falls. This makes Colt more suitable for northern regions, although you can always use frost protection.
Colt is resistant to phytophthora root rot, bacterial canker, and stem-pitting, although is susceptive to crown gall. Colt’s vigour is unaffected by viruses.
Gisela 5 shows a slight reduction in vigour when infected by viruses.
Primrose does also stock F.12.1 on a few trees, which is a vegetative clone of Mazzard. It produces a tree 6m tall, which is unsuitable for most gardens and takes up to 5 years to come into bearing.
Gisela requires 3-3.5m between trees, Colt 4-5m and F.12.1 6m.
Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!
His ideal afternoon would involve a long walk, before settling down for scones.
Jorge is a journeyman gardener with experience in growing crops.