Hi Michyb and welcome to GoY. Suckers are one of the ways plants have of producing new plants, and / or spreading wider. New plants are also produced by the plant making seeds, or runners. Broken-off bits of some plants root very easily which is another way of spreading.

This is a good exam question. Many answers to be provided. Beattie has one, I'll add some. Suckers are as B says, a vegetative way of reproducing. Often induced if the top of the plant is damaged, but suckers are also produced from roots which lie near the surface of the ground, and sometimes a good distance from the parent plant, especially if the roots are damaged, perhaps with a low cut with a mower. Other examples might be with grafted plants. A prime example would be the contorted hazel, which is so often poorly done so that the rootstock, which is 'ordinary' hazel, sends up straight shoots from ground level, or just below, ruining the shape of the subject. Cutting the straight stems out doesn't solve the problem, either. Carefully 'stripping' off the suckers, removing latent buds, is one method. Tree culprits for their suckering habit are Rhus, some prunus species, esp. wild cherry after root damage, white poplar, dogwood. However, the habit can be used to good effect if a copse or spinney is the desired effect, or a shrubby screen is required. Worthy

One plant to avoid which suckers profusely is Ailanthus altisima, the 'tree of Heaven'. It's an attractive tree which grows very quickly but it will send up suckers all around the base which cannot be killed completely. It also deters other plants from growing close to it, like many suckering shrubs and trees. The most notorious suckering shrub of all must be the bramble, as it's amazing how every time the shoot touches the ground it roots! And bamboo isn't much better with its underground suckers which travel metres and then shoot up as a new cane.

Many woody plants have dormant growth buds on both roots and stems. These are kept dormant because the hormone which initiates growth is prevented from getting to them. If something interrupts the flow of sap to the normal growing buds, then the dormant buds will begin to grow. The interruption may be caused by something as simple as a branch bending under its own weight. Or it can be something as catastrophic as an axe! Suckers occur when the dormant buds on a root receive a dose of growth hormone, usually after some sort of mechanical damage. Some trees and shrubs have roots which are more easily damaged and thus produce more suckers than others.

We found this out to our cost when we pruned down our Rhus. tree because of wind damage only to get over five hundred suckers going the whole length of our garden! It was a nightmare and we had to get help to dig up the the roots, it cost us a lot of money.

Just a quick p..s. The process whereby a Bramble for example spreads itself by rooting as it goes is not strictly speaking 'suckering'. It is better described as self layering.

In the wild, suckering is also a quick way for plants to regenerate after disasters like fires, floods, or landslides.