In drawing a landscape plan, always make the circles that indicate plants of such size that they represent the ultimate spread of the specimens. In fact, to play safe you might add an extra 6 to 12 inches of diameter.
Generally speaking, the spread of a plant is almost equal to its height, the exceptions being those plants that have a definite upright habit. Thus, to indicate a forsythia at an outer corner of your house where it would be best if allowed to grow at least 6 feet tall, you should show a circle 6 feet across your plan.
This means, of course, that the bush (at the center of the circle) will have to stand at least 3 feet from the building, porch, or walk. Although older forsythias grow much larger than that when location and space permit, as part of a foundation planting they can be pruned annually and be kept beautiful as 6 foot specimens.
Shrubs that can be trained into small trees are often useful combination plants in a foundation planting. An example is the widely distributed deciduous blackhaw (Viburnum prunifolium). While young it makes an excellent flowering shrub with a beautiful horizontally branched habit and handsome foliage right down to the ground. Placed 5 or 6 feet out from the corner of a house and given ample room to spread, it would be a perfect subject there for at least ten or twelve years, after which it would become a little too large for that spot.
However, you could then begin pruning off the lower branches and thinning out the top to make it a most attractive small tree that would not exceed 18 feet in height even after thirty years. A new corner planting could be developed beneath its branches. This is a good way to satisfy the impatience that most of us have in gardening we should choose plants that, like the black-haw, produce a desired quick effect, and then, years later when we have a few dollars to spare, can serve as part of a new planting.