While some  smaller and more rural graveyards still allow, or even encourage, the involvement of family members in the landscaping around a loved one's grave, many cemeteries today post signs that request that visitors do not plant permanent plants.  The reasoning behind this is that assuming the plants survive, over time they can easily become over grown if not cared for on a regular basis The growth of these plants can and will begun to cover up the gravestones, making it difficult for others to find the burials, or possibly causing damage to the stone itself.  Some types of plants can spread rapidly and not only cover the gravestone, but the entire area surrounding it. Lilacs in particular can really spread and take over a cemetery.  

Cemeteries as a horticultural repository

Cemeteries are not only memorials to the dead; they also have secured a vital function as horticultural repositories. An article in the November 1996 issue of Southern Living  discussed the cemetery as a storehouse of plants as opposed to a storehouse of bodies. Many of the plants found in older cemeteries reflect the horticultural tastes of a different era, and sometimes antique varieties of plants that are thought to be either endangered  or lost can be found growing in older graveyard. It should also be noted that in many cases cemeteries also functioned in the capacity of "testing grounds" for plants that are now common in our yards and gardens. Before any clean up or landscaping is attempted, you should make sure you aren't disturbing valuable or rare plant life. In some cases, it is a crime to remove live plants from a cemetery.

The trees that can be found in cemeteries may be some of the oldest and largest types of their kind in the area as they were to some degree protected from being cut down for what ever reason.