Very few of the genera from which ornamental plant shrubs derive cause as much excitement or as much consternation in the Southeast as the hydrangea. Containing more than 70 species, not to mention hundreds of varieties, cultivars, and forms, the magnificence and variety of its offerings, and their varied care, can be intimidating to old and new gardeners alike.
Of course, though the genus itself has members numbering in the hundreds, only a few species are commonly seen in the nursery trade in our neck of the woods. They include Hydrangea arborescens (smooth hydrangea), Hydrangea macrophylla (big leaf hydrangea), Hydrangea paniculata (panicled hydrangea), Hydrangea quercifolia (oak leaf hydrangea), and Hydrangea serrata (similar to big leaf but more compact). Though all of these plants are deciduous, and all like acidic soil to various degrees, they vary in average size of plants, leaf shape, and bloom shape, size, and color. But, alas, they all seem to share one common complaint — brown or black spots covering the leaves in the middle and late summer months, up until leaf drop in the fall.
I tend to think wood mulch is a bad idea for all plants, preferring pine needles, but it’s especially bad for plants like hydrangeas that are prone to fungal problems.
When this late-summer fungal funk appears, so too do customers begin appearing at retail garden centers, carrying leaves in bags, photos on smartphones, or both, wanting to buy something to cure this disease, to save their plants, and make them look pretty again, right away. The sudden repulsiveness of the leaves, contrasting with the striking beauty of the blooms, destroys what might otherwise be the best time of the year for some hydrangeas, July through leaf drop. Alas, by the time these symptoms appear, there is not much that can be done for the plant that year.
Oh sure, it’s possible a garden center employee might successfully identify the exact fungal strain attacking a particular plant, maybe Cercospora hydrangea or Botrytis cinerea, and sell the customer the right fungicide, that, if caught early enough in the season, might knock it out and minimize the damage. But, more likely, this effort will be an expensive and somewhat time-consuming fail. Instead, the best course of action is prevention, the steps of which should begin in the Mid-South in February.
Now, to be clear, these fungal infections, though terribly unsightly, are rarely directly fatal to the plants. Still, they may as well be because of the degree to which an infected plant detracts from the value of the landscape. So, what can you do to prevent this disease?
The steps are fairly simple. Since hydrangeas are deciduous and shed their leaves each fall, the leaves are typically re-infected each spring by spores that overwinter in the soil below the plant — spores that originated in last year’s leaves. So, the first step is good hydrangea hygiene. Gather up leaves as soon after leaf drop as possible and, when you enter your garden on the first not-too-cold day in February, be scrupulous in getting up any of last year’s fallen hydrangea leaves.
Second, get rid of any wood mulch from previous years under the canopies of your hydrangea leaves. I tend to think wood mulch is a bad idea for all plants, preferring pine needles, but it’s especially bad for plants like hydrangeas that are prone to fungal problems, being a perfect medium to overwinter fungal spores and hold moisture excessively at levels that are conducive to fungal problems.
Third, sprinkle a quarter cup of soil sulfur under each plant. This will serve two purposes. It will indirectly acidify the soil, and all hydrangeas prefer an acidic soil. Since all organic matter tends to revert to neutral over time, regular efforts need to be made to lower the pH of your soil. Hydrangeas absorb nutrients better when the soil is acidic and are healthier plants, better able to fight off pathogens of all kinds. But, soil sulfur also acts as a sporicide, a sort of birth control for fungi, reducing the degree to which fungal spores are likely to evolve into fungal bodies on the leaves of plants.
Fourth, put down a thick layer of pine straw. Hydrangea leaves are re-infected as they first begin to emerge in the spring. The powerful showers in March and April pound into the soil and ricochet the fungal spores up onto the new and tender leaves. Putting down a thick layer of pine straw creates a barrier between the soil and the new leaves, without the downsides of wood mulch.
Finally, make sure their other nutritional needs are being met by following a reasonable fertilization program, whether it be organic or synthetic. There are many approaches to providing plants with needed nutrients, but a simple one, if you don’t already have one, is 1/4 cup time-release 12-6-6 fertilizer in the spring and a 1/4 cup time-release 6-12-12 fertilizer in the fall, sprinkled under the canopies of the plants. When a plant’s basic nutritional needs are being met, it will be more resistant to pathogens of all kinds.
So, follow these instructions for spot-free, healthy leaves that will complement the beautiful summer hydrangea blooms. And, if you don’t have any hydrangeas, now is a great time to plant them!
John A. Jennings is an ISA Certified Arborist, and manager of horticulture for Palladio Garden in Midtown Memphis.
