It is winter in the city, and the streets are cold and gray. Tree pits that have been planted with evergreens, such as the buxus plants shown below, seem particularly attractive at this time of year. However, woody plants need more root space than annuals and so can be difficult to plant in small tree pits. New Yorkers are used to addressing confined space issues by building up, but this can be treacherous to do in a tree pit. Building up the soil in a tree pit more than 2-3 inches is not recommended because it can cause the tree bark to rot and fall away, exposing the trunk to disease. In addition, the additional soil can become compacted and prevent water and oxygen from getting to the roots below. So although the evergreen buxus shown below are vibrant green even in the dead of winter, this is at the expense of damaging the trees they are surrounding. The Parks Department does not recommend installing any solid walls around the tree pit that will retain soil; metal guards that keep dogs away but allow water and soil to drain are best.
A better way to bring color to winter tree pits is to lay evergreen branches over the tree pit. Evergreen branches can look and smell lovely without building up the tree pit and damaging the tree. Branches are easy to get and easier to "plant," and can actually be good for the tree. Evergreen branches make a good winter mulch, protecting the soil from the worst of the cold and road salts that can be damaging to trees in winter. Branches should be removed in the spring, but then there are probably more interesting things to done in tree pits as weather warms.