Many ornamental trees, shrubs, roses and climbing plants and most fruit trees are grafted or budded onto a rootstock of a compatible species. This is done for plants that are difficult to propagate from cuttings, where the rootstock is needed to control the rate of their growth or it imparts disease resistance.
Sometimes shoots appear from below ground level from the rootstock – which are known as suckers. Being shoots of the rootstock species, rather than the cultivated variety (scion), there is no advantage in keeping them and they should be removed. If left in place they can weaken the plant, completely take over, and may soon become a forest of suckers that take over areas of the garden.
Some plants are more prone to suckering than others, including lilac, false acacia (Robinia), Prunus species (cherries, plums etc), roses and sumach (Rhus).
Suckers from the rootstock can also appear when the graft union, where the cultivated variety and the rootstock are joined, fails. This is called graft failure and leads to the grafted variety breaking off the rootstock or, because it prevents the flow of water and nutrients from the roots, it dying back, poor growth or even death of the grafted variety. The rootstock usually remains alive and starts suckering.
There are several possible reasons why plants start suckering. These include poor formation of the graft union during the propagation stage, stress at the roots – especially from waterlogging, drought, wind damage (wind rock), cold temperatures and frost, pests and diseases among others – or physical damage to the graft union or rootstock.
Shoots appearing on the stems of standard roses and top-grafted trees are also from the grafted species, rather than the cultivated variety. These should also be removed – ideally when they are still small and can be rubbed off with a gloved hand.