The search for that ingenious, indisposable, enduring, environmentally safe Mother's Day present is over. And the answer is right in your own yard: a tree. You can honor your mother with a tree in your yard or hers. Or, depending on where you live, you may get your government to buy and plant one for you in the strip of grass between the sidewalk and street, or maybe even in a park.

In the district, you can plant your own tree next to the sidewalk, provided it's a species approved for your street and the city has designated a vacant space for it (282-2367). Tax-cutting has abolished Prince George's County's tree-planting program, but you can get a permit to plant trees yourself on public property (952-4270). Montgomery County has the best deal: place your order by January (remember this for next Mother's Day), and for $8, the county will plant one of six approved shade trees on county land, if the soil and space are suitable. Otherwise, you can request a permit to do the planting yourself (565-7588). The Maryland Forest Service also has a bargain for group tree-buying: order 10 trees (choices are mostly among oaks and maples) to plant on public space, and you pay only $3 to $7.50 each (776-4511 in Montgomery, 372-8128 in Prince George's). Virginia's Department of Highways has no special tree offers, but it will give advice on what trees can be planted and where (273-0660). CAPTION: Illustration, no caption, By Patrice Moerman