Experts and BHG readers answer.
How do I keep my plant from growing crooked towards the sunlight?
Lilacs are best adapted to growth in full sun. When grown in the shade, they won't flower well and, as you have noticed, may grow in the direction of brighter light. Short of removing the ash tree to create full sun, you may not be able to keep your lilac growing straight upright. Plant growth bending toward light is due to phototropism. All plants produce growth substances called auxins. Auxins promote stem elongation. The auxin on the sunny side of the plant is broken down by the sunlight, creating a higher auxin concentration on the shady side of the stem. The shaded side of the stem grows faster than the sunny side, resulting in a "bend" toward the light. You've probably seen this with houseplants. Unless you give the plant a turn now and then, all the leaves will soon be facing the window or light source. Because you can't very well give your lilac in the ground an occasional turn, the only way to keep it growing straight is to provide it with uniform light all around. If you prefer to keep the ash tree, a solution may be to replace the lilac with a shade-tolerant shrub such as hydrangea, fothergilla, or viburnum.