Tree seedlings can be a novel surprise when they germinate in the garden and are usually from something which is growing nearby. They are brought in by birds which have eaten their fruit, sometimes stashed away by squirrels for food, or some have special seed casings which allow them to float on the wind. However, most are from large trees which have no place alongside ornamental plants. The first seed leaves or cotyledons, can be similar for a number of species but the true leaves soon emerge and they closely resemble those of the mature tree. Usually they are Ash, Beech, Birch, Sycamore or Elder and if left they will become a nuisance later on - they should be removed as soon as possible, as if they grow more than about 20cm high they become difficult to uproot. They usually send down a strong taproot which stabilizes the sapling as it grows and it has a tenacious hold.
The saplings can easily be missed if they germinate in a hedge and unless it is a mixed native one, they will make an unsightly mess later. Specimens with large, deciduous leaves are particularly damaging as they shade out the hedging plant and in winter this leaves a gap. They usually produce faster-growing shoots than the rest of the hedge so it becomes untidy soon after cutting. Always check the base of the hedge when cutting it to remove the seedlings before they can do any damage. Examples of culprits are Elder, Lime and Sycamore. Ivy, though an evergreen, will overwhelm the hedge and kill it leading to its eventual collapse. There will usually be a few Brambles as well.
If this has occurred, prune out the unwanted branches to allow the hedging plant to grow back. If it is a large area leave some of the weed plant and gradually remove it as the hedge recovers, so as not to make large holes. It may not be possible to remove the root, but if the stump is continually deprived of any shoots it will eventually die away - or it could be treated with a root-killing herbicide. A spike or crow-bar driven down vertically beside the stem of the young tree should loosen it enough to allow it to be pulled out without disturbing the roots of the hedge too much.
Some seeds germinate in old walls or chimneys and the roots grow through the soft mortar forcing the bricks or stones apart. Typical examples are Cotoneaster, Ivy and Buddleja.
Close to an Ash or Sycamore tree there may be hundreds of seedlings which are time-consuming to uproot. If it is a clearing they can be removed with a line trimmer when they still have seed leaves with soft stems, or with a weedkiller. Saplings can be treated with Vitax SBK Brushwood killer which contains the selective herbicide Triclopyr or stronger Glyphosate based products such as Bayer garden tree stump killer, Growing success Deep Root ultra and Roundup Tree stump and Root killer.
Follow these links for further details on Weeds, Weed Removal and Weed Prevention.