Pests and diseases 67
temperature fluctuations, such as warm dry days and cool nights. Close spacing
of plants can contribute to decreased air movement, so providing a bit more
room for the plants and keeping them away from walls or thick hedges can help
decrease problems. Thinning of mildew-prone perennials is often recommended
to improve air circulation within the plant’s structure, although with phlox I
have not noticed any reliable difference between thinned plants and unthinned
plants—both seem to get mildew equally well if the conditions are right. It seems
that selecting resistant forms is the surest approach.
Other mildew-ridden perennials include species of Pulmonaria and Chrysogo-
num, which are often subject to attack in dry locations. Plants can be cut to the
ground—sometimes new clean growth will be already evident under
mildew-ridden foliage, but not always—and fresh clean leaves will emerge.
The foliage of ‘Nice Gal’ peony
(top) holds up pretty well
through the season compared
to many other peonies, such as
‘Festiva Maxima’ (bottom),
which exhibits the unsightly
decline of many peonies.