Powdery problems

I have three peonies. Two are fine, but the third, in a different location, is completely covered with powdery mildew. How does that happen? Should I do something about the one that’s covered? Or just leave it and hope for the best?

Powdery mildew (which affects all kinds of plants in the landscape) can be a problem when weather conditions are right and cultural conditions are less than perfect. It’s a fungus that thrives in warm weather when the humidity is high. It becomes more of a problem for plants that are growing in damp, shady places and overcrowded conditions.

It’s a common disease and you’ll know when it hits: look for patches of gray-white, powder-like growth. It usually appears on the tops of leaves but can also be seen on the bottoms of leaves, and on young stems, buds and flowers. It likes the young, succulent parts of plants.

At the UT Extension Soil, Plant and Pest Center, expert Alan Windham (who frequently provides answers to questions here) says the peony should survive with no problem. It’s a good idea to remove any dead or dying foliage and destroy it (don’t put it in the compost; that probably won’t kill the fungus spores), and clean up around the area.

Windham is more worried about downy mildew in beds of impatiens, which we mentioned in this column several weeks ago. “There are lots of cases coming in from all over the state; it’s been found in nearly every state east of the Mississippi,” he says.

Watch for plants that are losing leaves, that don’t flower, and that have white growth on the undersides of the leaves, he advises. The disease can be extremely damaging, so pull up, bag and dispose of infected plants to keep it from spreading.

Bad news this year, but even bigger implications for next year regarding availability, use by commercial landscapers and their general viability as a bedding plant, he said. Does that mean the ubiquitous impatiens won’t be among the gardener’s favorite go-to shade annual next year?

“Begonias, SunPatiens and New Guinea impatiens are going to be in high demand,” he says.

If you haven’t seen the Soil, Pest and Plant Center’s Facebook page, check it out here. “We’re putting lots of good stuff up,” Windham says.

Garden events in MiddleTennessee

Sept. 5:WarnerParkNatureCenter hosts Hummingbird Happy Hour, 4 – 6 p.m. at the WarnerParkNatureCenter. Bird banding demonstrations and other kid-friendly activities are on the agenda. Free admission, but registration is required; call 352-6299. While you’re there, visit the NatureCenter’s vegetable, herb and flower garden. Info: http://www.nashville.gov/parks/nature/wpnc.

Sept. 15: Urban Chicken Appreciation Day at Shelby Bottoms Nature Center, 10 – 11 a.m., celebrates feathered backyard friends. Learn how to get started in backyard chicken keeping in a session led by Bonnie Bowles. Call (862-8539) or email (shelbybottomsnature@nashville.gov) to register.

Sept. 22 & 23: Many rare and unusual houseplants will be available at the Tennessee Gesneriad Society’s annual Flower Show and plant sale, which will be held at Cheekwood’s Botanic Hall. The event is open to the public 9:30 a.m. – 4:30 p.m. Saturday, and 11 a.m. – 4:30 p.m. Sunday. To learn more contact Julie Mavity-Hudson at Julie.mavity@gmail.com.

Sept. 29: Herb Society of Nashville’s Herb Day at Cheekwood’s Botanic Hall: “Cooking & Gardening with Native Plants.” 9 a.m. – 4 p.m.; admission is $40 plus Cheekwood gate fee. To register: www.herbsocietynashville.org.

Evergreens turning ever-brown

QUESTION: We are seeing many evergreen trees — Leyland cypress and others — with big sections of brown limbs. Is that due to the drought? Or is there some other problem affecting the trees?

The drought this summer plays a big part in the browning of Leyland cypress, but it’s not the whole story. Alan Windham, at UT Extension’s Soil, Pest and Plant Center says the branch dieback is the result of a one-two punch: drought and seirdium canker, a fungus that appears on branches or stems and in branch axils and causes the branch to die. “I’ve seen more damage this month than I can remember in several years,” Windham says.

I found a good description of seirdium canker at the Web site of North Carolina State University’s Plant Pathology Extension: The cankers are brown or purple sunken patches on the bark, and may be accompanied by a flow of resin. Affected branches may be scattered randomly throughout the tree; they turn a reddish-brown color, in striking contrast to the green, healthy foliage. The fungus can be spread by splashes of rainwater or water from sprinklers, or it can travel from branch to branch on unclean pruning tools.

There are no chemicals recommended to control the canker. Brown branches should be pruned and destroyed as soon as possible. Prune at least an inch below the canker, and sterilize the pruning tools between cuts by dipping them in rubbing alcohol or a solution of 1 part chlorine bleach to 9 parts water. Plants that are severely affected should be removed and destroyed. Watering by drip irrigation during drought can help prevent problems, Windham says.

Upright arborvitae are also having trouble due to the drought. Windham explains: Plants have different strategies to survive: some plants sacrifice older leaves to protect new shoots; some have waxy leaves, some plants roll their leaves or close up to prevent water vapor from escaping.

“Then you have plants like arborvitae, where it’s all or nothing. It’s as if the plant is saying, ‘We’re going for broke. Everything survives or nothing survives.’ Well, this strategy didn’t work out too well for many arborvitae I have seen.”

In this case, the only solution is to remove the dead trees and start again.

Garden events in Middle Tennessee

Aug. 21: Two meetings at Cheekwood’s Botanic Hall:

- The Perennial Plant Society meets at 6:30, beginning with refreshments and plant swap. Program begins at 7 p.m., and the speaker is Audubon naturalist Sherra Owens, presenting “Gardenening On The Wild Side With Nature’s Most Important Plant.”

- The Orchid Society of Middle Tennessee meets at 7 p.m. Michael Wenzel of the AtlanticBotanical Garden will speak on Phalaenopsis species.

Both meetings are open to the public.

Aug. 23: The Middle Tennessee Hosta Society meets at Cheekwood, 6:30 p.m., in the Potter Room. Guest speaker is landscape designer, photographer and writer Troy Marden; his topic: In a Southern Garden: Lessons from 20 Years of Gardening in the South.” To learn more about MTHS, visit www.mths-hosta.com.

Next month in Middle Tennessee

Sept. 5:WarnerParkNatureCenter hosts Hummingbird Happy Hour, 4 – 6 p.m. at the WarnerParkNatureCenter. Bird banding demonstrations and other kid-friendly activities are on the agenda. Free admission, but registration is required; call 352-6299. While you’re there, visit the NatureCenter’s vegetable, herb and flower garden. Info: http://www.nashville.gov/parks/nature/wpnc.

Sept. 15: Urban Chicken Appreciation Day at Shelby Bottoms Nature Center, 10 – 11 a.m., celebrates feathered backyard friends. Learn how to get started in backyard chicken keeping in a session led by Bonnie Bowles. Call (862-8539) or email (shelbybottomsnature@nashville.gov) to register.

Sept. 22 & 23: Many rare and unusual houseplants will be available at the Tennessee Gesneriad Society’s annual Flower Show and plant sale, which will be held at Cheekwood’s Botanic Hall. The event is open to the public 9:30 a.m. – 4:30 p.m. Saturday, and 11 a.m. – 4:30 p.m. Sunday. To learn more contact Julie Mavity-Hudson at Julie.mavity@gmail.com.

Sept. 29: Herb Society of Nashville’s Herb Day at Cheekwood’s Botanic Hall: “Cooking & Gardening with Native Plants.” 9 a.m. – 4 p.m.; admission is $40 plus Cheekwood gate fee. To register: www.herbsocietynashville.org.

Too hot to bloom!

QUESTION: I have a pot of impatiens that was doing well, but now the flower buds have been dropping off or turning brown before opening. The pot is always in the shade and I keep it well watered and fed. It has nice green leaves, just no flowers. Any idea what could cause this?

If there are no other symptoms – no spotting of the leaves, no rotting of the stems, no powdery coating or other unusual growth – a good guess at the source of the problem would be the long period of extreme heat. Even in the shade it has been extremely hot (remember that day it hit 109 degrees?) and plants in containers may suffer more because the pot can dry out quickly. A plant under stress will shed its flowers first.

By now, with several days of rain and more reasonable temperatures, the impatiens (and many other things) should begin to recover.

This is a good time to mention again, though, an email I received from UT Extension plant disease expert Alan Windham, in which he warned about the development of downy mildew in beds of impatiens. Watch for plants that are losing leaves, that don’t flower, and that have white growth on the undersides of the leaves, he advises. This disease can be extremely damaging, so pull up, bag and dispose of infected plants to keep it from spreading. There’s more about it at UT Extension’s Soil, Pest and Plant Center Facebook page.

…And too hot to plant, too

QUESTION: My daughter has thinned out her iris bed and given some to me.  She has given me the whole plant (bulbs and stems). I know I can not plant them at this time but how do I store them until they can be planted in November or December?  Do I need to cut the stems off now or leave them as is?  Thanks for any information you can provide. 

You’re right, with too much heat and still too little rain, it’s a bad time to plant anything. Irises are pretty hardy so you can wait to plant the rhizomes.

For now, cut the leaves off to about 3 to 6 inches, and remove as much of the soil from the rhizome as you can. Don’t wash them, just brush off the dried soil. Check to make sure there are no rotting places, insects or diseased-looking spots (discard those if there are) and store them in a cool, dry place.

You don’t need to wait until November to plant them. While they’re pretty hardy, they’d still rather be in the ground, so consider planting in September, or after the worst of the summer heat passes.

When you do get ready to plant, here’s a helpful link from the American Iris Society that provides information.

Another reminder about watering: In spite of all the rain lately, we’re still experiencing severe drought conditions in Middle Tennessee. Young trees suffer most. If you planted trees this spring, give them a little extra attention to help them make it through this hot, dry summer. Click here to see the Nashville Tree Foundation’s guidelines and tips to help keep your trees healthy.

Events coming up

July 26: Farmer Jason performs at Family Night Out at Cheekwood, 6:30 p.m. The event is free with regular Cheekwood admission. www.cheekwood.org.

Aug. 5: The Nashville African Violet Club will meet at 1:45 at the Green Hill Women’s Center, 10905 Lebanon Road in Mt. Juliet. To learn more, contact Julie at Julie.mavity@gmail.com

Aug. 12: The Tennessee Gesneriad Society will meet at Cheekwood at the Frist Learning Center at 2 p.m. The program will be a polinaton/hybridization workshop. For more info contact Julie at Julie.mavity@gmail.com (or 615-364-8459).

Prevent powdery mildew

QUESTION: How do I keep my beautiful zinnias from getting powdery mildew? It may be too late for this year, but what should I do different next year?

Powdery mildew is a fungus that thrives when nights are moderately cool and foliage stays damp. It travels by airborne spores, and appears as gray or white splotches on leaves, stems and flowers of susceptible plants, such as zinnias. A mild case is merely unattractive; a severe case of powdery mildew can cause distorted shoots and leaves, misshapen flowers or can prevent flowering altogether.

The best defense is to give zinnias room to grow without crowding, which allows air to circulate better around the plants, and water only in the morning, so the foliage has time to dry before nightfall. Cut back on the use of high-nitrogen fertilizer, which produces succulent new growth that is a major powdery mildew magnet. Grow them in full sun; hot temperatures (above 90 degrees) inhibit the growth of mildew.

There are fungicides available that should be applied as soon as you begin to spot the mildew (so yes, probably too late for this year), but I always suggest trying the good-cultural-practices method first. The University of Tennessee Extension has a short list in a publication about powdery mildew here. If you decide to go that route, be sure to read and follow directions on the product label.

Zinnias are not the only things plagued by powdery mildew. Lilacs, roses, crepe myrtles and other woody ornamentals, and many herbaceous ornamentals and bedding plants are also targets when weather conditions favor the fungus.

Coming up

July 17: Middle Tennessee Orchid Society meets at 7 p.m at Cheekwood. On the agenda: an orchid auction and discussion on Cattleya Orchids. To learn more, visit www.tnorchid.com.

July 17: Perennial Plant Society meets at Cheekwood in Botanic Hall. Refreshments and plant swap at 6:30; meeting and program at 7. Guest is Kelly Norris, horticulturist, plant breeder and manager of Rainbow Iris Farm in Bedford, Iowa will present “The Modern Plant Explorer.” The meeting is open to the public.

July 26: Farmer Jason performs at Family Night Out at Cheekwood, 6:30 p.m. The event is free with regular Cheekwood admission. www.cheekwood.org.

The trouble with roses

The leaves on my climbing roses developed little black spots and now all the foliage has dropped off. What is this?

For questions on roses, I always go to the helpful experts at the Nashville Rose Society, and their Web site. It looks like there are at least two fungal diseases that cause black spots on the leaves of roses: one appropriately called black spot, and the other, anthracnose. You can tell the difference by looking at the edges of the spots. Black spot has the feathery margins, which give rise to some of its other names: leaf blotch or star sooty mold.

Both make a rose look really bad for awhile, which is why people who are serious about growing fancy, beautiful roses stick to such a rigid schedule of spraying. Fungicides are to ward off the ugly fungal diseases, pesticides to keep away chewing and sucking bugs.

Both blackspot and anthracnose overwinter on the plant and develop in the incubator of a cool, most spring. Cleaning up around rose bushes (getting rid of dead leaves and decaying matter), pruning out affected canes, giving the rose bushes plenty of air and reducing the amount of time water stays on the leaves can go a long way toward reducing the development of disease spores. Fungicides, applied on a regular schedule in early spring, can help prevent infection.

The problem of dealing with and preventing rose diseases is one reason landscapers and gardeners plant so many of the ‘Knock Out’ varieties, which bloom all summer and are resistant to most of the ugly diseases that plague garden roses.

Which leads to another question a friend asked not long ago:

Are ‘Knock Out’ roses real roses?

I understand the reason for the question, because it does seem impossible, doesn’t it? Given all that you hear about how you have to prune, spray and coddle rose bushes to get them to look their best, how can there be these easy-care upstarts, this ‘Knock Out’ series that seems poised to take over the rose universe? To get a rose expert’s view on the topic, I talked to Anne Owen, a Nashville Rose Society consulting rosarian.

 “They are, in fact, a really true rose, in the genus Rosa,” she said. “The fact that they don’t require spraying for fungal diseases puts them in a category with very few others.”

Most rose enthusiasts, she said, like the challenge of growing the big, show-stopping roses – the ones in striking colors and with exceptional fragrance. And those are the ones that require coddling. “If you want a great big cabbage rose or a big hybrid tea, you’re going to have to spray.”

But a ‘Knock Out’ finds a place even in the most serious rose gardens.

“I think most rosarians have a ‘Knock Out’ or two because they are such great plants,” Owen said.

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This month in the garden: Welcome the blooms, not the bugs: June Landscape & Garden Calendar in The Tennessean.