Yard & Garden Line News Volume 8 Number 7                                                               May 15, 2006

Features this issue: Maintaining the Blue in Blue Hydrangea Look for Viruses Before Purchasing Hostas This Year Gardening Calendar Asparagus Beetles Are Active Now Editorial Notes

Maintaining the Blue in Blue Hydrangea
Carl Rosen, Extension Soil Scientist

'Endless Summer'-pink and blue Photo credit: Bailey's Nurseries

The popularity of growing hydrangeas in Minnesota has increased dramatically with the recent release of the winter hardy zone 4 cultivar 'Endless Summer'. Like most hydrangeas, the flower color is blue when soil conditions are acid and then converts to pink when soil conditions become slightly acid to alkaline. During this conversion the plant may have flowers of various shades of blue and pink.

The blue and pink color of hydrangea flower sepals is due to an anthocyanin called delphinidin 3-monoglucoside. In the presence of aluminum, a blue color will form due to the aluminum binding with the anthocyanin. The reason for the blue color under acid conditions is due in part to an increase in availability and uptake of aluminum from the soil. As soil pH decreases, availability of aluminum increases. Conversely, as soil pH increases the availability of aluminum decreases. Ideally, the pH of your soil should be around 5.0 to ensure that adequate aluminum is available for blue flower color development in hydrangeas. Of interest is that aluminum is not considered to be an essential element for plant growth. In fact, for most plants, high levels of available soil aluminum are toxic causing stunting of root growth and eventual death if soil aluminum is high enough. Usually the soil pH has to be less than 4.5 for this to happen.

Gardeners in eastern U.S. have a difficult time maintaining pink flowers because their native soils tend to be acid and contain sufficient levels of available aluminum to maintain a blue flower color in hydrangea. In contrast, soils in the upper Midwest tend to be more alkaline, making it difficult to maintain blue flowers. In Minnesota, soil pH tends to be highest in the western part of the state where calcareous soils are prevalent and then becomes more acid in the east. Therefore, gardeners in the western part of the state will have a harder time maintaining hydrangeas with a blue flower color. For soils that are highly calcareous (pH 7.8 and higher), it may not only be a problem to maintain blue flowers, but the plant itself may exhibit yellow leaves due to iron deficiency. In those cases, growing hydrangeas in raised beds with a more acid soil mix may be the only solution.

Hydrangeas sold in pots at garden centers are usually grown in artificial potting media. For blue flower color, the medium is maintained in the acid range by using sphagnum peat moss and various acidifying amendments as discussed below. When the plants are transplanted into high pH soils, the pH in the root zone will eventually increase and flower color will turn pink unless steps are taken to maintain acid soil conditions.

The following procedures are suggested for maintaining the blue flower color in hydrangeas:

Before planting:

1) Have your soil tested for initial pH level. This can be done by sending your sample to the University of Minnesota Soil Testing Laboratory (see: http://soiltest.coafes.umn.edu/ for more information). Alternatively, you can try doing your own testing by using a soil testing kit purchased from a garden center. If you do your own testing, it is recommended that you split your first sample and send half to the University lab for pH determination. On the remaining half, do your own testing. If your results are similar to the lab results then you can assume that your pH tester is working properly.

2) If your soil pH is less than 5.5 the only amendment suggested before planting is to mix in sphagnum peat moss into your soil at the rate of 1 to 2 cubic ft per plant. (use a blend of 50% native soil to 50% sphagnum peat)

3) If your soil pH is greater than 5.5, incorporate elemental sulfur into the top 6 to 8 inches of soil at the following rates to reduce your soil pH by 1 unit:

For sandy soils - 1.0 lb per 100 square feet (about 2 cups)
For loamy soils - 2.5 lb per 100 square feet (about 5 cups)

For example, if the pH of your loamy soil is 6.0, incorporate 5 cups of elemental sulfur per 100 square feet (or ½ cup per 10 square feet). After mixing in the elemental sulfur, mix in 1 to 2 cubic feet of sphagnum peat moss per plant.

After planting:

1) Periodically retest your soil pH. Do not add any acidifying amendments if your soil pH is 5.0 or less.

2) To maintain good vegetative growth and acid soil conditions, use ammonium sulfate as the nitrogen source at the rate of 1 lb (or 2 cups) per 100 square feet (or a little less than a ¼ cup per 10 square feet). Ammonium sulfate is the best nitrogen source to help maintain soil acidity.

3) If your soil pH is greater than 5.5, use aluminum sulfate to help lower your pH and supply available aluminum at the same time. Note that aluminum sulfate is generally not recommended for lowering soil pH because high rates can injure most plants. Hydrangeas are the exception. Mix about 1 lb (2 cups) of aluminum sulfate per 5 gallons of water and then apply the solution around the drip line of the plant. Apply enough of the solution per plant to just saturate the soil. Repeat the application on a monthly basis as long as your soil pH is greater than 5.0. Caution - over application of aluminum sulfate can be toxic even to hydrangea. Monitor your soil pH periodically and only apply aluminum sulfate if your pH is greater than 5.0.

Non-acid loving plants growing in the vicinity of hydrangeas may not grow well if the soil pH drops too low. It is recommended that you leave a 2-3 ft buffer zone between your hydrangeas and plants that prefer higher pH conditions or place other acid loving plants such as azaleas, rhododendrons, and blueberries next to the hydrangeas.

Look for Viruses Before Purchasing Hostas This Year
Michelle Grabowski, Regional Extension Educator-Horticulture

As you are shopping for new plants for your garden this year it is always important to check plants for disease before bringing them home. Look for any leaf spots, soft water soaked areas on stems and leaves, or cankers that might indicate disease problems. If you are looking for hostas this year, there are two important viruses to be aware of, Hosta Virus X and Tobacco Rattle Virus (TRV). These are two very different viruses.

Hosta virus X Photo credit: Ben Lockhart

The symptoms of Hosta Virus X vary depending on the cultivar. The very light colored 'golden' plants show symptoms of dark green spots and mottling. A streaking between veins that has an ink bleed look to it is also common. Some hostas show dried dead steaks or spots and others show no symptoms at all. Only a few cultivars have been shown to be resistant to the virus (ex. 'Bressingham Blue', 'Frosted Jane', 'Great Expectations', 'Love Pat', 'Sagae', and 'Elegans'). At the nursery look for symptoms on plants you wish to purchase. If there is a group of the same cultivar of hostas from the same nursery, some with symptoms and others without, don't buy any of them. Hosta virus X can be present in a plant without showing symptoms. If some plants in a group are infected and displaying symptoms it is likely the plants without symptoms are infected as well. Now that Hosta Virus X is becoming well known, many nurseries are testing their plants for Hosta Virus X. Be sure to ask at your nursery if there stock has been tested.

If a hosta begins to show symptoms after purchase, Hosta Virus X can easily be removed from your garden by taking out and destroying all infected hostas. This virus can only be transmitted from plant to plant by transferring infected sap from one plant to another on gardening tools or through other injuries. Once the infected plant is destroyed, Hosta virus X cannot live. Wait 1-2 weeks for any remaining roots to die before replanting in the same spot.

Tobacco Rattle virus on hosta 'ananuma' Photo credit: Ben Lockhart

The symptoms of Tobacco Rattle Virus (TRV) include dark and light green mottling on the leaves, pale green spots that can die and become brown, necrotic spots. TRV is a European virus that is becoming more common in the United States as more and more ornamentals are sold internationally. Unlike Hosta Virus X, which only infects hostas, TRV has a wide host range including vegetables like beets, tomatoes, and spinach, as well as many ornamentals like tulip, daffodil, gladiolas, coral bells, bleeding heart, calendula, sunflower, and many more. TRV also infects common weeds like lambs quarter, black night shade, and pigweed.

Unfortunately TRV is much more difficult to control than Hosta Virus X. TRV is spread by a trichodorid nematode, a microscopic worm-like creature that lives in the soil and feeds on plant roots. Through these nematodes the virus can be spread from plant to plant. Chemicals to kill these nematodes are highly toxic and not commonly available to homeowners. Once in the garden the virus may infect multiple plants and be very difficult to get rid of. Prevention is the best form of control for TRV. In the nursery, hostas are typically infected with TRV through an infected stock plant and will almost always display symptoms. If you avoid hostas with viral symptoms you have a good chance of avoiding TRV altogether.

Buy only healthy symptom-free hostas this year. Neither of the two viruses above can be cured. If you suspect that a plant you have purchased may be infected with either of these two viruses, send several fresh leaves to the plant diagnostic clinic at the University of Minnesota (http://www.extension.umn.edu/distribution/cropsystems/DC3170.html). There is a $25 minimum charge for virus testing. If you suspect that the plant has Tobacco Rattle Virus, do not plant the hosta until you have confirmed its health.

Contributors: Nancy Rose Patrick Weicherding Bob Mugaas

Tulip swath Photo credit: Beth Jarvis

Lawns:
Mow as best you can between rain storms. It's stressful for grass plants if you lop off a good chunk all at once. If your mowing got away from you, cut the grass long, then gradually drop the mower height. Your lawn should not be yellow, from overly tall grass shading the grass below, when mowed.

If your lawn has a whitish cast when mowed, your mower blade is dull. Buy a second blade so you always have a sharp blade on hand. It's healthier for the grass and saves gasoline.

Newer lawns, especially those on sandy soil, will benefit from a half strength nitrogen application if conditions remain really wet. Nitrogen leaches easily from soil.

Creeping Charlie is at full bloom so now is the time to spray with glyphosate or triclopyr.

There's time to get a pre-emergent crabgrass application down. Pre-emergent should be watered in. However, if thunder storms are pending, hold off on the pre-emergent as it might just wash away.

Trees and Shrubs:
Now is a good time to plant containerized or balled and burlapped trees and shrubs. It's too late to plant bare-root materials. Be sure to locate the first lateral root and plant your tree or shrub so that first lateral root is at or slightly above the adjacent soil. For further enlightenment, visit: http://www.cnr.umn.edu/FR/extension/urban_com/plantingtreesshrubs.html.

Mulch, mulch, mulch but please don't pile mulch up the trunks of trees--AKA "volcano mulching".

Now is a great time to prune to correct storm damage, remove dead branches or for control of diseases such as plum pockets and black knot. Do not prune members of the rose family: apples, honeylocust, cotoneaster, pears and mountain ash as you could spread the fire blight-causing bacterium. It is spread by rain, insects, wind and birds. It over-winters in cankers on infected trees. Heavy spring pruning encourages succulent growth that is more susceptible to fire blight infection. Prune these plants in the fall or dormant season.

It is safe to prune arborvitae, yews, juniper, and hemlock if you're pruning to control height. Hold off until first week of June to prune pine and spruce. You will want to prune only the new growth.

Start scouting for insects and diseases. June is often prime time for sawflies--those green caterpillars that feed heavily on evergreen needles and some deciduous trees. Rose slug sawfly will start ravaging rose bushes starting right about now. The sawfly pub is at: http://www.extension.umn.edu/distribution/horticulture/DG6703.html

Be cautious with broad leaf weed control products--especially those containing dicamba in the active ingredients such Trimec.


Fruit, Flower, Vegetables:
Fourlined plant bugs may be out and about. They feed on the chlorophyl in plant leaves and leave depressed, rounded spots on leaves. They'll attack a wide variety of plants, including weeds. Read all about them at:
http://www.extension.umn.edu/projects/yardandgarden/EntWeb/QuizArchives/2005/quiz-0505.html
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/e121plantbugs-fourlined.html

As rhubarb sends up flower stalks, remove them. They serve no purpose.

Scout your irises for water-stained streaks or patches on the leaves that may indicate iris borer presence. Cygon is no longer available, so read about your options at: http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/e122irisborer.html

Petal-fall is a key time for spraying fruit to minimize coddling moth and plum curculio damage (http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-May0102.html#apple).

The lion's share of the vegetable and flower gardens can be planted now, at least in the southern half of the state. Peppers need warm soil, so wait until Memorial Day or later to plant them. Cool temperatures set them back. Northern Minnesota gardeners should be able to plant the bulk of their gardens by Memorial Day. Succession planting, at two week intervals, of sweet corn, salad greens and beans could start now.

Direct seed all herbs but basil which needs to go into a warm, sunny spot. Herbs don't need fertilizer.

As you weed the emerged vegetables, you may begin to mulch. Mulched soil remains cooler so wait until the soil has warmed. Cool season crops may be mulched.

Fertilize your spring-flowering bulbs before the foliage dies back. Deadhead (remove) seeds heads of tulips, daffodils and others as flower petals fall or fade. Do not remove the leaves until they have died back. Leave daffodil foliage undisturbed; don't braid or bunch them. Foliage shoulld be left to senesce (die back) naturally, routing food manufactured in the leaves to the bulb.

Asparagus Beetles Are Active Now
Jeffrey Hahn, Assist. Extension Entomologist

Common asparagus beetle Photo credit: Jeff Hahn

The most important insect pest of asparagus in home gardens in the common asparagus beetle, Crioceris asparagi. This beetle has a slender body, is 1/4 inch long, has a reddish prothorax (the area behind the head) and has metallic blue-black wing covers with red along the edge and three pairs of yellowish or cream colored spots on each side (sometimes two spots may merge and appear as one).

Common asparagus beetles are active in early May (in central Minnesota) or about the time asparagus is first coming up. Their feeding can be damaging to the emerging spears and can cause browning and scarring. This feeding can also cause asparagus spears to bend over, a injury referred to as a shepherd's crook. You may also occasionally see the spotted asparagus beetle, Crioceris duodecimpunctata. You can identify this related beetle from its 1/4 inch size, its red-orange color and the twelve black spots on its wing covers. This beetle is active a few weeks after the common asparagus beetle is first seen.

Asparagus beetle eggs Photo credit: Jeff Hahn

During May, asparagus beetles lay rows of very small, dark-colored oval eggs on the asparagus. Common asparagus beetle larvae are dark-colored and slug-like. Once they hatch they move to the ferns to feed. The common asparagus beetle is potentially more damaging at this stage because it can defoliate the ferns. Larvae feed for two to three weeks then pupate. Adults emerge and feed on ferns for the remainder of the season.

You may also see spotted asparagus beetle larvae which are also slug-like but are orange in color. They are far less damaging as they generally feed just on the berries. After the larvae pupate, adults emerge and feed on ferns until late July.

The best time to manage asparagus beetles is in early to mid May when they are first active and before too much damage has occurred. Unfortunately, we are generally past this time already for this season. If you have an ongoing problem with asparagus beetles, remember to check early for their presence next spring as soon as asparagus spears start to emerge. Asparagus beetles are most active in afternoon so check then to get an accurate idea of how numerous they are. If you find few or no beetles, control isn't necessary. However, if you find beetles on one out of ten or more plants, then you should take steps to manage them.

Also watch later on for asparagus beetle larvae. Fortunately, small or moderate numbers of larvae do not usually defoliate the ferns enough to damage plants and require management. However, large numbers of beetles could severely defoliate and injure asparagus.

If you need to manage adults or larvae, first consider physical removal. This is particularly effective you have a small garden. Just take out a pail of soap water and knock the adult beetles or larvae into it. You can also physically remove the eggs that you find. While they are small, they are fairly easy to see upon close examination. While look for eggs, keep your eyes open for a tiny (1/16th inch long) metallic green wasp. This parasite (parasitoid) attacks asparagus beetle eggs. If you find them in your garden preserve them by not spraying insecticides.

However, if you find it is necessary to spray asparagus beetles, there are several active ingredients available to treat them. Common examples include: permethrin (e.g. Bonide Eight Garden & Home and Ortho® Bug-B-Gon® MAX® Garden Insect Dust), malathion (e.g. Bonide Malathion), Cyclopropanecarboxylic acid, 3-(2,2dichloroethenyl)-2,2-dimethyl-,(3-phenoxyphenyl) methyl ester (e.g. Bonide Bug Beater® Yard & Garden Ready-to-Spray ), or carbaryl (e.g. Bayer AdvancedTM Complete Insect Killer for Gardens Ready-To-Use).

Not all garden insecticides list asparagus on the label. Read the product label carefully before buying it to be sure the insecticide is labeled for asparagus or any other plant you wish to treat.

Get the low down on this month's insect pests at Insects
http://www.extension.umn.edu/projects/yardandgarden/EntWeb/Ent.htm
Editorial Notes

'Dordogne' tulips, Japanese maple and snapdragons. Photo credit: Beth Jarvis

Next issue, I'm going to start the "tree-legalitlies" articles courtesy of Lorrie Stromme, attorney and master gardener. The series will cover herbicide drift/spray issues, utility pruning, property line tree issues, and much more.

Dave Hanson, forester and the guiding light for the Shade Tree Advisor course, will be writing an article some time this summer on growing evergreens in shade. I've asked him to touch on the physiological drawbacks to shade vis a vis incidence of winter injury, among other things.

Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.

Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/.

Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Friday of every month at 10 a.m. The program is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations. (Scroll down for map.)

For plant and insect questions, visit http://www.extension.umn.edu/askmg. Thousands of questions have been answered, so try the search option in the black bar at the top left of the board for the fastest answer.

If you would like to receive an e-mail reminder when the next issue of the Yard & Garden Line News is posted to the web, just send an e-mail to: listserv@lists.umn.edu (note: the second E in listserve is omitted), leave the subject line blank, then in the body of the message, type: sub yglnewslist or to unsubscribe, enter: unsub yglnewslist

Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator


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