Now that the weather is heating up (hopefully this last weekend in May will be a mere hiccup with night temperatures in the thirties and forties), I’d like to recommend some shrubs to you, Physocarpus (Ninebark), (Hydrangea quercifolia (Oakleaf Hydrangea), and Kerria japonica (Japanese Kerria) that are relatively impervious to the heat.
Physocarpus opulifolius is as tough as can be. My ‘Diabolo’, a purple foliaged cultivar, receives almost full sun all afternoon and is never watered except by Mother Nature. It is a very large, vase-shaped shrub that needs a space 10’x10’ unless you want to prune annually. Use it as a multi-season screen that has attractive foliage, ruby-red buds that open to pale pinkish-white flower clusters in May, and red fruit from June until fall when the foliage turns bright reddish-orange.
There are, supposedly some smaller cultivars such as ‘Center Glow’ and ‘Summer Wine’ but not having grown them, I can’t attest to their smaller stature over a period of time. I have grown a relatively new dwarf cultivar called ‘Donna May’ (First Edition Little Devil™) that is just as tough as ‘Diabolo’ and seems to be a bit more shade tolerant
I find that Physocarpus grows best in full sun. In shaded conditions, it survives but also seems to be susceptible to powdery mildew.
How does anyone live without at least one Hydrangea quercifolia? This versatile shrub will grow almost anywhere except poorly drained, constantly wet soil. The large, oak-leaf shaped foliage lends strong textural contrast to small-leaved shrubs and perennials. The species grows 4-6’ x 3-4’ and is an excellent background plant.
Its large, pyramidal, white panicles bloom in early June and frequently remain on the plant through the winter when the exfoliating bark is quite visible. In the fall, foliage turns shades of orange to maroon, depending on the amount of sun received.
There are several cultivars, my favorite being ‘Snowflake’ that has double flowers that are absolutely gorgeous but also heavier than the normal panicle and thus the branches often have a fountain effect instead of being upright.
There are also three dwarf cultivars with which I am acquainted. ‘Peewee’ and ‘Sikes Dwarf’ have the usual green foliage and white flowers and only grow to three feet and could, therefore, be used in the middle of the border.
‘Little Honey’ is a yellow-leafed cultivar that burns in the sun but is a chartreuse standout in partial shade.
Do not prune unless there is some dieback or you want to encourage a bushier form. If you need to prune, do so immediately after bloom or you will lose the blooms for next year; unfortunately, it also means that you will be cutting off some of this year’s blooms.
I’ve grown Kerria for at least twenty years in a dry shady spot under some privets that were fifteen foot trees when I moved into my house thirty years ago. The first Kerria I grew was ‘Golden Guinea’ which has single yellow flowers. There is another cultivar, ‘Pleniflora’ that has double golden yellow flowers that I hate. It’s merely a matter of personal taste.
Once I discovered ‘Picta’, I took out ‘Pleniflora’ and replaced it with ‘Picta’. The variegated green and white foliage lights up the shade and I also prefer the single, truer yellow flowers. They all bloom in April and May. Both cultivars and the species have stems that remain evergreen throughout the winter. Kerria also grows quite well in the sun but the flower color washes out. It will also bloom quite well in nearly full shade.
The beauty of all these shrubs is their tolerance of varying conditions. Although very drought tolerant, once established, they also sailed through 2011’s wet conditions with no problem. Want to beat the heat? Try these three