Repeated cycles of bloom over the growing season, bright scarlet or yellow flowers, and being a butterfly magnet all come together to make this South American native a sought after plant by gardeners. Although tropical milkweed is technically a perennial sub-shrub in cold hardiness zones 8-11, it grows quickly from seed and can serve as an attractive annual in Minnesota. What really sets it apart from other milkweeds is that it continues to flower throughout the growing season, becoming more and more impressive as the season continues. The original stem of a new seedling terminates in flowers. Side, or axillary branches then emerge and flower. This process continues causing the plant to become increasingly branched and contain both new flowers and splitting, floss and seed-filled capsules typical of most milkweeds. Deadheading encourages faster rebloom. By fall plants attain ~3-4’ in height and ~2-3’ in width making tropical milkweed a suitable back of the border or specimen plant or attractive accent in a large container. The most common cultivars are ‘Butterfly Red’ (Scarlet-orange and yellow bicolor flowers) and ‘Silky Gold’ (yellow flowers).

Tropical milkweed grows best in full sun and moist, yet well-draining soil. A hard frost will kill plants. Although seed production is abundant, natural reseeding the following spring has not been observed in the University of Minnesota Display and Trial Gardens. Plants come relatively true from seed and seed can be collected and stored cool and dry indoors over winter for planting the next season. Heavy natural reseeding, however, is possible for this species in Southern states and has limited it use.

University of Minnesota alumnus, Greg Nordwig (advisor was Dr. John Erwin, Floriculture Physiologist), developed a production schedule for tropical milkweed so greenhouse growers will have the information they need to successfully produce this crop. It was demonstrated that flowering plants can be obtained in about 15 weeks from seed using standard bedding plant protocols, a timeframe similar to many of the other spring bedding plants such as geranium (Pelargonium × hortorum). Although tropical milkweed is still catching on as a new crop for Minnesota bedding plant producers and be in limited supply in garden centers, home gardeners can start their own transplants. Seeds can be obtained through specialty catalogs and can also be found on seed racks at some local garden centers.

The growth and flowering requirements of tropical milkweed is pretty simple. They are day-neutral, meaning that photoperiod (day and night length) is not a cue for flower initiation and development, unlike plants such as chrysanthemums and morning glories (short days/long nights trigger flowering). This simplifies production because plants will faithfully flower under various photoperiods. Flowering will occur faster, however, if plants are grown under higher light intensity or longer days because of increased growth rates.

If you are planning a trip to the Minnesota State Fair and you would like to see tropical milkweed, you should be able to find some growing in pots in the butterfly house. It serves as food to many species of butterflies and due to its repeat flowering nature is commonly used by those that cultivate butterflies. Tropical milkweed is a beautiful, versatile, and easy to grow plant with great potential for Minnesota landscapes.

---

The 2008 Minnesota State Fair is just around the corner! Opening day is Thursday August 21 and it will run through Labor Day, September 1st. This year promises to provide a great array of horticultural exhibits, hands-on demonstrations, and vendors. Please stop by the booth of the University of Minnesota Master Gardeners within the Ag/Hort building and ask your gardening questions and join in on daily demonstrations. On display within the building will be many kinds of vegetables, fruits, and flowers. Check the schedule to know what will be going on each day.

Some of the flowers that will be featured include African violets, gladiolus, orchids, and dahlias. Venders will be on hand selling Minnesota produced honey, fruit, and other products. In addition, throughout the fairgrounds are many wonderful gardens sponsored by businesses and garden clubs. Locations and a brief description of these gardens can be found here.

Come join in on the fun!

---

Marssonina leaf spot on Euonymous

This leaf spot disease has been seen in several locations on the small native tree, Euonymous atropurpurea, often called eastern wahoo. It is a close relative of the common landscape shrub burning bush (Euonymous alatus). The disease can easily be recognized by purplish red spots with a cream colored center. Spots can occur on the leaves and young green branches of the tree, and are caused by the fungi Marssonina thomasiana.

Not much is known about the lifecycle of the fungi Marssonina thomasiana, but it is known that other Marssonina fungi are spread on splashing water from rain and irrigation. The first leaf spots form from spores created on last year’s infected leaves and branches. Each leaf spot then produces an abundance of new spores that are splashed to a new leaf and create more leaf spots. In this way a few leaf spots quickly turn into many leaf spots. This year’s infection rate is high and several hundred spots have been recorded on a single infected leaf. Some trees are so severely infected that leaves are browning and falling off.

Since the Marssonina fungi thrive in wet conditions, sprinklers should be directed to avoid wetting the leaves of nearby wahoo trees. Optimize air movement around the tree by pruning nearby plants that are crowding the tree. Unfortunately, little can be done about the early summer wet weather that has encouraged Marssonina this year. This fall, rake up and remove fallen leaves from around the base of the tree to reduce the amount of fungi able to survive to the next season. In addition, inspect young green stems and prune out any infected branches.

Nothing can be done to cure existing leaf spots. Well established trees will tolerate leaf spot infections. Newly transplanted trees or trees that have suffered from severe leaf loss more than two years in a row as a result of the disease can be protected with a broad spectrum fungicide like chlorothalonil or thiophanate methyl. These sprays will only protect healthy leaves and therefore are most effective if applied in early summer before disease sets in. Always read and follow all instructions on the fungicide label.

Fire Blight

Another disease being reported in Minnesota this summer is fire blight. Fire blight is caused by the bacteria Erwinia amylovera, and is a problem in members of the Rosaceae family. This disease can infect all members of the Rosaceae family including hawthorns (Crataegus), mountain ash (Sorbus), and brambles like raspberries (Rubus). Infection on apple, crabapple and mountain ash has been most commonly reported in Minnesota this summer.

In other states fire blight is a major concern in the spring time, especially during apple bloom. The fire blight bacteria can enter the natural openings in flowers, initiating disease that spreads down into the tree branches. Luckily in Minnesota, spring temperatures are often too cool for the fire blight bacteria to be active during blossom.

Summer infections, however, can be a problem when conditions are favorable for disease. The fire blight bacteria can infect trees through minor wounds created by strong storms, hail, insect feeding and pruning cuts. The bacteria thrive in wet or very humid weather at temperatures of 80-84F. Consistent rains combined with storms strong enough to whip branches against one another or even to generate hail, have created a favorable environment for summer fire blight infection this year.

Branches infected by fire blight have wilted brown leaves that often remain attached to the branch. The infection often progresses from the tip of the branch into the tree. In apple trees, the infected tip curls over in the form of a shepherd’s crook. Sometimes only one branch within the canopy is infected. In very susceptible trees, many branches may be infected resulting in a scorched look.

The fire blight bacteria move from infected shoots into branches and even into the main trunk of the tree. A discolored area of bark can be seen on infected branches, and the bark may look blistered or cracked. These infections are called cankers. If cankers girdle the main trunk of the tree, the tree can be killed.

Although it is difficult to prevent new fire blight infections, existing infections can be eradicated through proper pruning. Look for the discolored bark indicating the edge of a canker and make the pruning cut at least eight inches below this point. It is best to mark infected branches and make pruning cuts during the winter months to avoid the possibility of spreading the bacteria into the new wound. If the tree is very susceptible to the disease and the infection is moving quickly, or if it is close to the main trunk of the tree, pruning cuts can be made during the growing season to stop the progression of the disease. Cuts should be made during dry weather. Pruning tools should be sterilized with 10% bleach or a household disinfectant like Lysol or Listerine between every pruning cut!! Infected branches should be burned, buried, or otherwise destroyed.

---

While it seemed to take a long time to get here this year, summer did finally arrive. For our lawns that means a return to periods of hot and dry conditions. While this year has seen a lower number of 90+ degree days than typical, there have been and still are areas where rainfall has been slightly to well below average, resulting in many lawn and turfgrass areas turning brown. Our lawn grasses which perform best during the cooler periods of spring and fall adapt to hot and dry conditions by significantly slowing down their growth rates or even turning brown and going into a summer time dormancy condition. Normally, once rainfall (or irrigation) begins to increase and we get into the shorter and cooler days of late summer and early fall, these grasses begin to green up and resume more active growth. In fact, the amount of late summer – fall growth of our lawn grasses is second only to the active period of growth in the spring (assuming other conditions are favorable for active growth.)

There are, however, limits to how much stress our lawn grasses can endure before permanent injury and death result. If your lawn is one of those with significant injury and is in need of repair, mid-August through about mid-September is the best time of the year to do some overseeding to repair those damaged areas. At this time the soils are warm and seeds germinate much more quickly than in cold spring soils. In addition, there is much less competition from weedy grasses such as crabgrass or yellow foxtail. This gives our lawn grasses an excellent opportunity to germinate and get established before winter.

When doing overseeding, make sure that there is good seed to soil contact. This helps ensure more uniform germination and early seedling growth. Where all of the grass has died in an area, scratch up the soil surface with a garden rake and remove any excess dead grass. With the soil loosened, sprinkle some grass seed over the area and lightly rake it in. For mixtures of Kentucky bluegrass and fine fescue, seed at a rate of about 3 to 4 ounces per 50 square feet. If a blend of Kentucky bluegrass is used, seed at a rate of 2 to 3 ounces per 50 square feet. Always be sure to use fresh seed. Discard seed that may have been saved in the garage or basement that is more than two seasons old. It’s simply not worth the risk of poor seed germination and growth after you have spent the time to properly prepare the area.

After seeding, keep the area damp but not saturated with water. Germination should begin in a week or two. Continue keeping the newly seeded areas moist until you are about ready to mow for the first time. Gradually, increase the interval between waterings such that the soil dries out slightly before watering again. This will help harden off the grass going into the winter months. The first mowing should occur when the initial grasses to germinate and grow have reached about 3 inches. At that time cut back to about 2.0 or 2.5 inches. Continue cutting until leaf growth has essentially stopped for the year, typically about late October.

If overseeding into an existing thin lawn, keep the existing grasses mowed fairly short until the newly seeded grasses have had a chance to germinate and catch-up with the other grasses. If the existing grass is allowed to grow too long, they will compete significantly with the new seedlings often resulting in their death and your overseeding efforts will end in failure.

Your new lawn area will usually benefit from a late season application of nitrogen fertilizer. Again, in the Twin Cities area, this should go down at or just prior to Halloween. However, never apply fertilizer to frozen ground.

A little bit of overseeding and some good fall lawn care will help restore your lawn to a healthy, dense condition and be in a much better position to resume active, vigorous come next spring.

---

For larger views of most images, just click on the image.

Back issues Yard & Garden Line News for the past nine years are online.

For plant and insect questions, visit Ask a Master Gardener. 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.

Receive an e-mail reminder when the next issue of the Yard and Garden News is posted to the web:

Name

Email

unsubscribe

Happy gardening!

David C. Zlesak, Ph.D.
Editor
Extension Educator