The choice of rootstocks for pear trees is more limited than it is for apples, reflecting the fact that pears are not as commercially important as apples. Whereas rootstocks for apples and plums come from closely related Malus and Prunus species respectively, the situation with pear rootstocks is more unusual. The most widely-used rootstocks for pears in Europe come not from another pear species, but from a different species altogether - Quince (Cydonia oblonga). However, as is obvious from the shape of the fruit, there is still a close relation between quinces and pears which means that pear scions can be successfully grafted on to quince rootstocks. Quince trees tend to be smaller trees than pear trees, and quince rootstocks are also more precocious than seedling pears - so the resulting tree bears fruit at an earlier stage in its life than a pear tree on its own roots, usually within 3 years.
One side-effect of the more distant relationship between pear species and quince rootstocks is that not every pear variety is compatible. Where this happens an "inter-stock" or "interstem" has to be used, and the resulting tree consists of three parts - the quince rootstock, the inter-stock (which is often a pear variety such as Doyenne du Comice which has good compatibility with quince) and then the fruiting variety or scion. However it is thought that the slight incompatibility with quince is also important in the size control and precocity these rootstocks induce.
In North America, the most popular rootstock series is based on a cross between two pear varieties, Old Home and Farmingdale. Different forms have different vigours, but all have good cold-hardiness and fireblight resistance, which are important characteristics in the US climate. Being Pyrus in origin, these "OHxF" rootstocks do not have the latent incompatibility between pear and quince which is inherent in the Quince-derived rootstocks. However, these rootstocks are generally not used in Europe.
Although pears cannot be grafted on to apple rootstocks, some pear varieties can be grafted on to an apple rootstock via an interstem of the apple variety known as Winter Banana which seems to have some compatibility with pears. Perhaps even more surprisingly, it is possible to graft pear scions directly on to hawthorn roots (Crataegus), reflecting the fact that pears and hawthorns are both members of the rose family (Rosaceae) as are apples, plums, and cherries. However hawthorn rootstocks are not really suitable for pear trees used in garden situations.
Not surprisingly, Quinces are always grafted on to quince rootstocks. The lack of incompatibility between the scion and the rootstock means that quinces are generally a bit larger than pears on a given rootstock.
If you have heavy chalk soil then it is best to avoid quince rootstocks.
Quince C
Pears grafted on to the Quince C rootstock produce the smallest pear trees. The height after 5-10 years will be about 2.5m to 3m or so, or up to 2m when trained as a fan or espalier.
Quinces grafted on Quince C are likely to reach 3m, or 2m if trained as a fan.
Not suitable for heavy chalk soils.
Quince Adams
The Quince Adams rootstock (not to be confused with Quince A rootstock) is widely used by European growers but not well known in the UK. It is functionally similar to Quince C, but marginally more vigorous. It is not compatible with Williams.
Not suitable for heavy chalk soils.
BA29
Another dwarf rootstock for pears which is widely used in Europe, less so in the UK. It is not compatible with Williams.
Not suitable for heavy chalk soils.
Quince Eline
Eline is a new quince rootstock which can be used for pears and quinces, with similar vigour and productivity to Quince C. It was developed in the 1990s in the Netherlands, primarily to provide improved cold-hardiness over the widely-planted Quince C rootstock. Trees grafted on Quince Eline have a somewhat more erect habit than the same varieties grafted on Quince C or Quince A. Quince Eline also seems to reduce the extent of russeting on Conference, which is important for commercial growers.
Not suitable for heavy chalk soils.
Quince A
Quince A (not to be confused with Quince Adams) is the most widely-planted semi-vigorous rootstock for pears in the UK. Pears grafted on to the Quince A rootstock produce trees with a height after 5-10 years of 3m-4m / 10ft-14ft or so.
This is larger than the same variety on Quince C but not dramatically so. Quince A is often considered is less vigorous than the apple MM106 rootstock - it is at the lower-end of the semi-vigorous class and sometimes considered a vigorous semi-dwarf.
Quinces grafted on Quince A would reach 3.5m - 4m, or up to 2.5m if trained as a fan.
Quince A does best in good soils. It is not as tolerant of chalk or drought conditions as the more vigorous pear rootstocks.
OHxF 333
The OHxF rootstocks are based on a cross between the Old Home and Farmingdale pear varieties. This combination is naturally resistant to fire blight, and also cold-hardy. In the "333" form it produces a semi-vigorous tree of about the same size as Quince A, and roughly half the height of a seedling pear tree. It is popular in the Pacific North West of the USA. It is not used in the UK.
PyroDwarf
From its name it is often assumed that PyroDwarf is a dwarf rootstock, but it is actually a large semi-vigorous rootstock, equivalent to the apple MM111 or cherry Colt rootstocks. It is so-named because it is slightly more dwarfing relative to a full size pear seedling rootstock.
PyroDwarf produces a tree which larger than Quince A, and 50% - 70% of the size of a seedling pear tree, so perhaps 4.5m or around 16ft, or about 3m as a fan in good conditions.
PyroDwarf is a good alternative to seedling rootstocks, as it produces large attractive trees which nevertheless start bearing quite young, usually after 3-4 years. Being of Pyrus origin, there are no graft incompatibility issues with this rootstock.
Customers are sometimes alarmed by the lack of roots on PyroDwarf rootstocks - compared to quince rootstocks (or apple or plum rootstocks) a young PyroDwarf rootstock tends to look more like a carrot than a rootstock - a thick stubby root with little or no root hairs. Perhaps as a result of this strange root shape, it can sometimes seem a bit slow to establish, so do not worry if growth seems slow in the first year after planting (as long as there is at least some growth) because it will probably get going in the second season.
PyroDwarf is able to tolerate chalk / alkaline soils better than quince-derived rootstocks.
PyroDwarf was developed from a cross between Old Home and Louise Bonne pear varieties, but has only inherited some of the former's fireblight resistance.
If you can live with its vigour and initial reluctance to start growing PyroDwarf is one of the best pear rootstocks now available.
OHxF 87
The Old Home x Farmingdale rootstock in this "87" form produces a semi-vigorous tree of about the same size as PyroDwarf, and roughly two-thirds the height of a seedling pear tree. It has good winter-hardiness and blight resistance, and is widely planted in the USA.
OHxF 97
In this "97" form the Old Home x Farmingdale rootstock produces a full-size tree of around 20ft.
Pyrus communis (Kirchensaller)
This Pyrus seedling rootstock is used for producing large specimen pear trees. It is compatible with all pear varieties and eventually produces a very large tree - well over 6m / 20ft, and usually very long-lived. It can be grown on most soils and is tolerant of chalk.