I planted some beautiful Cotinus coggygria ‘Grace’ in practically full southern exposure in my yard, and their foliage has gradually turned from deep wine red to a pale olive green. Is my soil not acidic enough? Did I overwater? Or is it something else?
You are not alone in noticing this color change. I found the following discussion about Cotinus coggygria x Cotinus obovatus ‘Grace’ on a Houzz forum. Here is an excerpt:
“The true ‘Royal Purple’ smoke bush will not turn green. Some smoke bushes are grafted, most do not grow true from seed. You must order one from a true nursery or visit a specialty nursery to get a good Royal purple smoke bush. There are also many different purple smoke bushes who have different characteristics. ‘Grace’ is a hybrid cross between the green smoke bush and Velvet cloak (which is a gorgeous true purple smoke bush). ‘Grace’ has blue green leaves with red tinges around the leaves.”
An article by David Wheeler in the British paper The Telegraph describes this variety of Cotinus:
“A hardy plant of exceptional beauty at several different times of the year, Cotinus ‘Grace’ has nothing of the leaden darkness so often criticised by those who dislike purple foliage. Less purple than red suffused with green, the leaves bring a bold note to the garden that can complement or offset other foliage colours.
‘Grace’ is a garden hybrid, one of five seedlings resulting from a deliberate cross between the American chittamwood (Cotinus obovatus) and a variety of the Venetian sumac (C. coggygria ‘Velvet Cloak’). Raised by Peter Dummer, propagator at Hillier’s nursery, in Winchester, in the late 1970s and named after his wife, it has won the Royal Horticultural Society’s top awards. Like both parent plants, it exhibits long-lasting autumn colour, the prominently veined, broad oval leaves (up to 5in long) turning to a brilliant scarlet before falling in mid-October.”
A page from Woodlanders Nursery catalog suggests the green phase is normal:
“Reddish new foliage turns blue-green and finally good orange, red, and yellow in fall. Plant in a sunny location with well-drained soil.”
The Brenton Arboretum in Dallas Center, Iowa describes the leaf color change similarly:
‘Grace’ – Hybrid between C. coggygria and C. obovatus; large pink panicles; leaves 4 to 6″ long, light red when young, turning blue-green in summer and red, orange or yellow in autumn; height estimated 20′; zone 5.
It sounds to me as if the leaves go through several changes, and your patience will probably be rewarded with the expected fall color in time.