I have a purple smoke tree that is about 9 years old. It is a large, healthy tree, but it has never “smoked.” Every spring it is full of blossoms, then when other smoke trees are beginning to smoke, the blossoms on this tree shrivel and die. I have tried watering it more, I have tried watering it less. Any ideas? J.W., Ann Arbor
Although smoke tree or smoke bush (Cotinus coggygria) requires well-drained soil and full sun, it is quite adaptable to different soil conditions. A large shrub or small multi-stemmed tree that can grow to 15 feet tall, it can work as a background plant for the mixed border or as a single specimen plant. Its claim to fame is the cloud of pinkish gray “smoke” that covers the plant in the summer. The effect is the result not of flowers, but of tiny pinkish hairs on the small fruits that occur in large clusters. There are some popular cultivars with purple leaves and darker flower clusters, such as ‘Royal Purple,’ ‘Velvet Cloak,’ and ‘Nordine Red.’
While smoke bush is relatively easy to grow, it can use some help in the early stages of development. For a new plant, avoid planting in waterlogged or poorly draining soil. For an existing plant, dig a dozen holes with a crowbar at the drip line that are a foot deep. Fill these holes with 5-10-10 fertilizer. This should be a one-time application. Early spring is best but it can be any time. It isn’t necessary to repeat this unless the smoke bush is not thriving in later years. Woody plants are going to be happier if you don’t overdo it. Annuals, tropicals and turf are heavy feeders, but over-fertilizing perennials and woody plants can force them to produce more stems and leaves than they want to, which can lead to unnecessary stress. Once established in the landscape, shrubs and trees adjust to the nutrients in the soil and often don’t want too much extra pampering. Avoid fertilizing (directly or indirectly) with high nitrogen (lawn) fertilizer; nitrogen promotes stem and leaf growth at the expense of flower production.
If your tree is planted in the lawn, remove the grass in at least a 6-foot diameter area around your tree. Mulch the area (do not pile mulch up against the trunk) and water infrequently: a good soaking once per week is much better than multiple small waterings. Smoke bush is drought-resistant and overwatering can result in few or no blooms, and makes it more susceptible to disease. Woody plants need watering less frequently than tender annuals, lawn, or herbaceous plants. Most established trees and shrubs can go weeks without supplemental watering except in very hot or windy weather.