Interest In Perennial Gardening Is Growing Year By Year

By Jan Howard

Local gardeners’ interest in perennial gardening has grown dramatically in the last few years, especially in Connecticut. Because of this, gardeners need to learn what steps to take to keep their plants coming back year after year.

Dr Sharon Douglas, a plant pathologist with the Connecticut Agricultural Experiment Station in New Haven, presented a talk and slide show on Common Diseases of the Perennial Garden April 4 at the C.H. Booth Library.

“The focus on perennial use has increased in the last five years,” she said. “Connecticut is the leader in the northeast with an 8 to 15 percent increase a year. There are new perennials every year.”

Dr Douglas’ responsibilities include diagnosis of plant health problems for all Connecticut residents, including homeowners, professional arborists, commercial growers and landscapers, and owners of golf courses and orchards.

Inquiries number about 7,500 a year. Seventy-four percent of the samples are from homeowners.

Her presentation was an overview of common problems encountered in perennial beds. She discussed the recognition and management of diseases, and disease prevention.

She said perennials are relatively problem free due in part to the diversity of plants, which helps to minimize chances for widespread disease outbreaks. “It is not often that we have outbreaks of diseases, but you want to know which diseases you can live with,” she said.

Gardeners need to be able to accurately identify disease problems when they occur, she said. It is important to recognize which diseases are aesthetic rather than life threatening so appropriate controls can be implemented.

Disease Prevention

By following a few basic measures, many diseases of perennials can be avoided or minimized.

Spacing and depth are important when planting, Dr Douglas said. “Anticipate the size of the plant at maturity. They need air movement around them.”

Planting at the depth recommended is important for winter hardiness and flowering.

Appropriate applications of fertilizer maximize plant growth and vigor and help avoid plant stress.

“It is important to strike the right balance,” Dr Douglas said.

Properly applied mulch helps with weed control, soil temperature, and soil moisture retention. Allow for a buffer zone between the plant and mulch, she said. Mulches should not be too thick so they retain too much water around the plant.

Water perennials to maintain adequate soil moisture. This is usually one inch of water per week, which is best delivered as a deep soaking. Water early in the day to allow foliage to dry before night.

For winter protection, mulches can protect plants from heaving during freeze-thaw cycles. They should be applied after the ground has frozen and removed before or when new growth starts.

Consider hardiness in selecting plants. This is usually not a problem with most perennials but should be considered when trying a new species. Connecticut is in USDA Zone 6 (some Zone 5).

When planting, pay special attention to soil type and pH, drainage, and light level. “If they are put in the wrong spot, they will be subject to disease,” Dr Douglas said.

If resistant or tolerant species are available, they should be selected. “They are not widely available now,” she noted, but efforts are being made to develop plants resistant to most common diseases.

Purchase healthy, vigorous plants and carefully inspect the root system before planting, she advised.

Remove infected plants and debris promptly to minimize disease spread. Also, during the growing season, remove spent flowers and leaf debris.

“Clean equipment so you don’t spread the disease,” Dr Douglas said.

Key Diseases

 An accurate diagnosis is needed to determine the type of control and if there is a need for control.

“You need to know the nature of the problem, the level of disease, and how to manage the problem,” Dr Douglas said. “We can learn to live with some of the diseases. We’re not growers, but we have to learn what levels are manageable.”

Key diseases are problems found on many types of plants each year. These include foliar diseases, such as leaf spots, blights, rusts, powdery mildews, and downy mildews.

Leaf spots are the most prevalent, appearing as dead areas on the leaves. They are host specific and are more common in wet weather, Dr Douglas said.

Blights are characterized by sudden damage, and are often more severe in wet weather. Entire leaves, stems, and flowers may be killed.

Early symptoms of rust infections appear as pale yellow spots. They are usually host specific and do not spread from species to species. “You don’t see many hollyhocks without this,” she said.

One of the most common diseases is powdery mildew. Symptoms appear as pale green irregular patches that gradually develop into white to grayish growth. They develop in July and August when it is very humid. Severely infected leaves quickly turn brown and drop.

Downy mildews are often misdiagnosed. Pale green or yellow areas appear on the leaves. Heavily infected leaves turn brown and shrivel in a very short period of time.

Fungicides are helpful, but in some of these cases need to be applied before symptoms appear.

“Not all fungicides are the same,” Dr Douglas said. “You need to know what you’re trying to control.”

Root, stem, and crown rots often produce fairly non-specific above-ground symptoms. Plants turn yellow, wilt, and droop, and can also appear stunted and have poor vigor.

“Plants under stress are prone to this,” she said. In extreme cases entire plants die.

Fungicides can be effective. Accurate diagnosis is necessary to select the appropriate one.

Vascular wilt symptoms include loss of rigidity, wilting, yellowing of foliage, drooping, and gradual dieback. The symptoms indicate water stress since the fungi and bacteria invade the plant’s vascular system. Fungicides are not usually effective. Use of resistant species and rotation are often the only effective control.

 Viral diseases can be very difficult to diagnose or detect. Symptoms include mosaics, mottles, ringspots, and distorted growth habits. Once the plant is infected, little can be done. Pesticides are ineffective once infection has occurred. However, they can reduce the spread of the disease.

Symptoms of phytoplasmal diseases are similar to those of viral diseases. Infected flowers will revert to leafy tissues, she said.

“Prevention is critical,” Dr Douglas said. Severely symptomatic plants should be removed.

Fact sheets regarding plant disease can be obtained from the Agricultural Experimental Station from its office or on the Experiment Station Web site: http://www.state.ct.us/caes.

Plant samples can be submitted in person or by mail.

For information, contact Dr Sharon M. Douglas, Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, 123 Huntington Street, PO Box 1106, New Haven, CT 06504.