Q My neighbor's old American holly tree overhangs most of both our narrow rowhouse backyards. Wrong plant for this place, obviously, but beyond that it has dumped leaves and dead twigs all year long for the last several years, and dropped berries continuously, beginning in October. Leaf drop used to be largely confined to May, and berries stayed on until late winter unless the robin conventions got them. The tree was professionally pruned and thinned about a year ago, and it has become a major nuisance. What ails it?
A Leaf drop is often caused by environmental factors rather than disease. Many broadleaved evergreens were scorched by the long period of cold weather last winter, and many of the scorched leaves dropped gradually throughout the growing season. Berry drop may be explained by environmental factors as well, including poor pollination.
Thinning and pruning are not necessary for hollies, and only result in rank growth that is more vulnerable to infestation by scale insects. American hollies are beautiful trees in their right place, but they do shed their prickly leaves, especially in May, and they cast dense shade.
It is within your rights to remove limbs that overhang your property, but I think this is a mistake -- not necessarily good for the tree or for neighborly relations. Offering to replace a problem tree often works, particularly if you are an avid gardener with a well-tended landscape that your neighbor admires.
Our privet hedge, about 40 years old, is beautiful when it leafs out in the spring but by late May develops brown spots and leaf drop. In the past we have sprayed with a fungicide that helped a bit, but we are trying not to use bad chemicals anymore. We used horticultural oil and the hedge was gorgeous, until the leaves started falling off by the bucketful. Also, what can we do about the powdery mildew on our phlox?
The fungal disease that you are dealing with is fairly common on privet. Frequent shearing makes the disease worse by limiting air circulation around the foliage. Privet needs to be renewed periodically be cutting it to the ground. In this way, you'll stimulate new growth that is more likely to resist the disease and improve air circulation within the hedge.
While it may be heartbreaking to see emptiness where your hedge once stood, the privet will grow very rapidly to its former stature. If you simply cannot bear the thought of doing this, you can thin the hedge by removing twiggy branches. Cut them back to a large structural branch. Remove enough of the stems to allow the internal branch structure to become just barely visible.
While some fungicides may help to prevent the disease, there aren't any organic controls for this disease.
The powdery mildew on the the phlox is a similar problem. The worst affected plants are in areas of poor air circulation and high humidity caused by frequent rain or watering. Phlox may be cut back if it is severely infected with powdery mildew. If it is not too late in the season, the plants often grow and bloom again. If you need to replace the phlox, look for a variety named David, which is resistant to powdery mildew.
What is the beautiful roadside planting of yellow flowers that appears in the fall and lasts until November? They grow to four feet, with leaves that are serrated, like a chrysanthemum's. Also, can you identify a white daisylike flower also popular in highway medians?
The yellow daisies you are referring to may be Rudbeckia laciniata, or cutleaf black-eyed Susan. The white ones are probably Shasta daisy, Chrysanthemum maximum. The former is native to our area, the latter is native to Eurasia. Cutleaf black-eyed Susan is perennial, and may sow itself to some extent if it is growing in moderately fertile, moist soil. Shasta daisy is perennial as well, but tends to die out if it is not divided periodically, and it is not as tolerant of excess moisture as cutleaf black-eyed Susan. Both can be sown directly in well-prepared soil that is free of weed seed. The best time to do so is in late summer or early fall, when the soil has cooled. You can purchase seed of Shasta daisy from your favorite garden center or from many online sources. You can find cutleaf black-eyed Susan at the mail-order catalogue Prairie Nursery (www.prairienursery.com or 800-476-9453).
Scott Aker is a horticulturist at the U.S. National Arboretum.
