Yard & Garden Line News Volume 6 Number 17                                                               November 1, 2004

Features this issue: Cold Acclimation: How plants stay warm in the winter Horticultural Turkeys Brahminy Blind Snake Pick Up Minnesota Gardening Calendar for 2005 November Garden Tips Editorial Notes

Cold Acclimation: How plants stay warm in the winter
Grace Pietsch, graduate student, Horticultural Science

Gaura blooms. Photo credit: Neil Anderson

With the cold weather upon us, I know I am bundling up in sweatshirts, coats, hats, etc. but what about the plants in our landscapes, lawns, roadsides, or other places? They are unable to throw on that extra jacket to keep warm, and they definitely cannot come inside to escape the cold, yet they still manage to survive most years.

So how are plants able to withstand our brutal temperatures and survive the winter? First let's look at what happens to many plants as winter approaches. I have set up a series of photos taken weekly during the months of October and November 2003 from the research fields of the University of Minnesota Flower breeding and Genetics lab in St. Paul. The plant in the photos is Gaura lindheimeri 'Siskyou Pink', which does not always survive the winter here in Minnesota. As you look at the pictures, note how the plant first stops flowering, and then begins to die back to the crown. We had a warm spell early in October last year which brought about the late flowers, however once the temperature began to drop, the leaves turned red and the stems died back. When the first snowfall came at the end of November, the plant looks almost dead.

Click to launch Powerpoint

By the time these photos were take, it was well into fall and the plants should have been able to withstand a fair amount of cold temperature. Factors which influence winter survival include plant dormancy and cold acclimation.

Oct. 9, 2003 Avg.  min. temp=45º Avg. max. temp=86º All photos: Grace Pietsch

Let's first take a look at dormancy. Dormancy is a condition where the plant or plant structure ceases to grow, or cannot grow. Two types of dormancy are involved in winter survival: eco- and endo-dormancy. Eco-dormancy describes a condition where environmental factors cause cessation of growth. Once these factors become favorable again, growth resumes. This can be seen in the first Gaura photo where temperature increased and the plants began to flower. Many herbaceous perennial species show this type of dormancy. Endo-dormancy is more complex, being regulated by physiological factors within the plant. Often photoperiod (daylength) is involved in the induction of dormancy. Most woody trees and shrubs have this type of dormancy. Winter survival and dormancy have been studied extensively in alfalfa (Medicago sativa), a perennial used in forage crops. Researchers have found that plants which go dormant earlier in the season are more likely to survive the winter. They speculate that what is happening to make the dormant plants more hardy is the accumulation of sugars and non-structural carbohydrates in storage organs (tap root in alfalfa). Plants don't necessarily have to be dormant to reach the next stage of winter survival.

Plants go through a process called cold acclimation, which is a transition from a cold-sensitive, actively growing phase, to a cold-insensitive, often dormant phase. Not all species can acclimate to cold temperature, even when given conditions where most plants do acclimate. Species such as potato (Solanum tuberosum) and tomato (Lycopersicon esculentum) will often incur freezing injury when temperatures become too cold, but cannot acclimate to the cold and will eventually die. Other species will acclimate, but the lowest temperature they can survive is higher than that seen in an average winter in Minnesota. An example of this is the Guara lindheimeri variety from the photos. This plant is native to southern Texas and can grow as far north as Indiana and Iowa, but will not always survive here in Minnesota.

Acclimation can be separated into two steps which are influenced by photoperiod (daylength) and temperature. How do photoperiod and temperature influence cold acclimation? Extensive research has been done at the University of Minnesota to determine how these factors affect woody ornamentals and fruit trees. Decreasing photoperiod is perceived in the leaves, and when the critical daylength is reached, sugars, starches, and proteins begin to accumulate in the roots. With this increased accumulation comes an increase in cold hardiness of the plant, along with dormancy as seen in alfalfa. In woody ornamentals, the decrease in daylength also initiates an increase in concentrations of calcium (Ca2+) inside the cells. This accumulation may be the trigger for the plant to increase cold tolerance and induce dormancy. This increase in Ca2+ has not been shown in herbaceous perennials.

Nov. 13, 2003 Avg.  min. temp=22º Avg. max. temp=59º

The critical daylength needed for plants to begin acclimation will vary greatly, depending on where the plant is originates. Woody ornamentals from southern latitudes will not begin to acclimate until much later than those from northern latitudes. If you think about it, in Minnesota, our daylength is much longer in the summer than southern states such as Texas or even Kentucky. The decrease in daylength from midsummer to the equinox in September is much more drastic (15 hr to 12 hr) than down south (14 hr to 12 hr in Texas) and continues to shorten at a faster rate as winter approaches. The plants originating from southern areas are less sensitive to the changes in daylength because they do not have the drastic changes we see here and do not acclimate as early. Thus, when buying plants from a southern nursery, be aware that they may not survive, or will have increased amounts of winter injury due to later acclimation.

The final stage of cold acclimation is induced by cold temperatures, which lead to the greatest level of cold tolerance. Some woody ornamentals require frost as a cue to acclimate to the greatest extent, while others only require exposure to a cold temperature above freezing. Most herbaceous perennials only require a cold period that is above freezing. Once cold acclimation is complete, plants can tolerate temperatures much lower than they would when un-acclimated. This does not necessarily mean they can tolerate temperatures as low as we find here in Minnesota, and care should be taken to find plants that are native to areas with similar climates to be sure they will survive.

Recently much research has been focused on the genes that control cold tolerance. Many genes appear to increase cold tolerance by a few degrees, but not much is known about how this is accomplished. Some of these genes seem to be involved in changes in cell membrane permeability. The model plant for most of these studies is Arabidopsis thaliana, a small plant in the mustard family. Genes have been cloned from Arabidopsis and put into many other species such as strawberry, corn, and tomato. Many genes that are similar to the Arabidopsis genes have been found in these other species as well.

Another focus of current research examines the sugars that accumulate in the roots of cold acclimated plants. Raffinose has been identified as a sugar that only accumulates when the plant has been cold acclimated, and the amount of raffinose present can be used to indicate relative hardiness of the plant. Much of the current alfalfa breeding projects are using this sugar as an indicator for cold tolerance.

In spite of all the research over the years to determine how plants "stay warm" through the winter, we still have a lot to learn. Today we are advancing by leaps and bounds with all the genetic information and technology available to us, but we still have a long ways to go before we completely understand cold acclimation and why the Gaura won't survive here in Minnesota.

Horticultural Turkeys
Deborah Brown, Extension Horticulturist

Wild turkey. Photo credit: Don Brennerman

I wrote this for the Star Tribune, back in 1993. Unfortunately, the same questionable products are being foisted off on unsuspecting gardeners, so I thought it bears repeating.

This isn't going to be a traditional Thanksgiving garden column. There'll be no talk of fall colored mum plants or dining table centerpieces featuring home grown squash, gourds, popcorn and Indian corn. We won't even dwell on the fact that what passes for yams at the supermarket is really a dark fleshed variety of sweet potatoes, and that most people wouldn't recognize a true yam if they saw it.

In honor or Thanksgiving, let's focus on turkeys, the "turkeys" of the garden world. They're the unusual products and ideas that just don't pan out. Some seem too good to be true while others sound fairly reasonable, but are a waste of money here.

* One such example is the "ever-popular" 5-in-1 apple with five different varieties grafted onto the same tree. The idea seems excellent for gardeners who want to try several kinds of apples but only have small yards in which to grow them. The problem is, most apple cultivars offered this way are only marginally suited to our climate. After a few years all but one or two cultivars will have died or become overshadowed by the dominant (best suited) one.

If your space is limited, it would be much smarter to plant several dwarf Minnesota apples. Not only are they compact, they begin bearing fruit at a very young age. And, because of their size, they are easier to prune, spray, or bag.

* Another perennial "turkey" is the tree tomato or "tamarillo," a woody member of the tomato family that's advertised in glowing terms each spring. It takes several years for a tree tomato to begin flowering and fruiting, but unfortunately, it's far from hardy in our climate. You'd have to grow it in a large pot and carry it indoors before frost every fall.

Tamarillo ad.

After struggling with the plant for several years, it might actually flower and bear fruit. But you'll probably be disappointed. Though tomato-like in appearance, they're not even close to tomatoes in flavor. In New Zealand where tree tomatoes are popular, people slice them in half, sugar them, and scoop out the flesh with a spoon. Or they peel and seed them, then use them in cooking.

* Anyone who's had trouble with their lawn has probably been tempted at some time to order zoysia grass plugs through the mail. There's a good reason local nurseries and garden centers don't handle it. Even though zoysia survives our winters, it performs very poorly in northern climates.

Zoysia plugs can spread and take over your lawn as ads suggest, but this happens slowly here over a period of many years. The real problem is not winter hardiness, but the fact that zoysia is a "warm season" grass. It doesn't green up till quite late in spring when temperatures warm, and it turns brown and goes dormant early in autumn as temperatures cool. Bluegrass lawns will be green and growing much earlier in spring and much later in autumn, at times when zoysia looks sick or dead.

Replacement guarantees are of no value. They won't send back the money, only more plugs to plant. And guess who pays the shipping and handling each time?
(Read more about zoysia at: http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Apr0102.html#zoysia

* Watch out, too, for any kind of "miracle plant" that's supposed to repel pests such as insects or rodents. There's no sound evidence that they're of any value above and beyond their perceived ornamental value. Special coleus, the newest plants in this category claim to repel cats and dogs – and they do smell terrible when you rub their leaves. But I've seen no evidence that they really keep animals out of your garden.

Euphorbia lathyris, sometimes called "gopher spurge" or "gopher purge" is supposed to keep destructive burrowing critters out of your garden. The plants' roots are toxic to moles and pocket gophers, but wildlife experts suggest that just means they won't eat those roots. It doesn't guarantee the animals will be repelled from the entire area. The same may be said for ultrasonic devices that are supposed to eliminate pests by subjecting them to sound waves that drive them away. Would that it were so easy!

Remember, think twice before spending your hard earned money on any of these turkeys!

Brahminy Blind Snake
Jeffrey Hahn, Assist. Extension Entomologist

Brahminy blind snake. Photo credit: Kenneth Krysko

A very interesting creature was discovered in the Minneapolis-St. Paul area in October. A renter discovered a dead worm-like animal on the floor of a ground-level apartment. She was worried that it was a parasitic worm and showed it to her pest control company. They did not recognize it and brought it to the University of Minnesota for identification.

The specimen was about 2 1/2 inches long and dark brown. In general appearance, it looked like a millipede but did not to have any legs or other appendages. It also looked somewhat like an earthworm but lacked any obvious rings or segmentation. Under a dissecting microscope, the body was discovered to be covered with a scaliness that looked similar to a snake. Upon closer examination, opaque-looking eyes were found and what looked to be a very small mouth. The animal was a snake.

The specimen was submitted to Tony Gamble, a Ph.D. student in the conservation biology program and a snake expert, to identify to species. He identified the specimen as a juvenile brahminy blind snake, Ramphotyphlops braminus. This is a tropical snake, originally from southeast Asia.

As commerce, especially greenhouse plants, have moved across the world, this snake has also traveled to new sites and is now found in all tropical areas in the world. The brahminy blind snake has even become successfully established in south Florida and Hawaii. This snake can be transported to more temperate areas in the U.S. in the soil of greenhouse plants. Because of this, this snake is also known as the flowerpot snake.

The brahminy blind snake is reputed to be one of, if not the smallest snakes in the world. Adults grow to be about five to six and a half inches long. This snake is parthenogenetic, i.e. only females exist. Egg develop without being fertilized by sperm from a male. Up to eight offspring can be produced. Newly hatched snakes are about two inches long. Brahminy blind snakes are found in soil and under leaves, logs and other debris where they feed on ants and termites.

Three additional brahminy blind snakes were found later in the apartment were the first was discovered. Their size suggests they had just recently hatched. They were undoubtedly moved into the building in a potted plant. Fortunately these snakes are harmless and control is unnecessary. It's possible that they could survive summers here in Minnesota but they would not survive winters in Minnesota unless they were in a greenhouse.

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

Pick Up Minnesota Gardening Calendar for 2005
Deborah Brown, Extension Horticulturist

2005 calendar. The 15th Year!

A gentle reminder: The holidays are just around the corner. Keep in mind, our new 2005 Minnesota Gardening calendar makes a great present for all your gardening friends and relatives. It's the perfect host or hostess gift at Thanksgiving, too.

This special anniversary edition of our calendar is chock full of good garden and landscape advice, gleaned from the past fifteen years. It includes a special feature on hardy ornamental and edible plants introduced by the University's Experiment Station since 1991, along with climate maps and information about horticultural organizations that serve northern gardeners.

You can find Minnesota Gardening, 2005 at gift and bookstores, garden centers, and extension offices throughout the state. You can also order it easily on line at http://www.extension.umn.edu/catalog/item.html?item=08101. Or if you prefer, call the extension distribution center, 612-624-4900 (Metro) or 1-800-876-8636 (Greater Minnesota) for credit card orders.

November Garden Tips
Beth Jarvis, Yard & Garden Line

Mums at Arb Photo credit: Beth Jarvis

These tips are based on conversations with Bob Mugaas, Patrick Weicherding and Bob Olson, Regional Extension Educators.

Trees:

Be sure evergreens are adequately watered from now until the ground freezes!

Lawns:

Leaves should be raked off lawns or chopped up finely to fall amid the grass. Leaves left on the grass block sunlight from the grass. For grass plants that were growing beneath tree limbs, this is the first crack at sunlight they've had for a long time. Don't block it.

If your grass has been getting cut at 2 1/2", you can drop the mower to 2" for the last fall mowings. Likewise, 3" lawns can go to 2 1/2".

Unless Novemeber turns unseasonably dry and warm, lawns should be able to get along on precipitation.

Dormant seeding should wait until mid month for TC metro, though northen Mn. could do it now. The soil must be very cold when the seed goes in and is watered.

Fruit:

Strawberries should be mulched with straw soon. Wait until the ground starts to freeze before you mulch. The ground typically freezes by mid-Nov. Meadow hay, coarse straw, sorghum-soudan grass are all aceptable as long as they're clean (weed-free). Raking leaves from the yard is not advised as the leaves will mat too much.

Apples: if you haven't cleaned up windfalls from entire orchard, do it. This will reduced the likelihood of disease and insect problems next year.

Mow under all fruit trees at a fairly close mowing as it will reduce harborage for rodents.

If you've not protected your fruit trees from voles, mice and bunnies, install that 1/4" hardware cloth cylinder now! White perforated tube type tree guards can go on first.

If deer are a problem, consider fencing orchard area to reduce mid-winter browsing damage. Thiram-based browsing retardents are effective and should be applied mid-winter, December, and possibly repeated if deer pressure is heavy. Some products have an offensive odor.

Vegetables:

Clean up and clear out the mortal remains of this year's veggie crops. You can till compost into the garden now to make room in the composter.

Flowers:

Tender bulbs, such as dahlias should be dug and stored now, if they've still in the ground.

Tulips can still be planted but other spring flowering bulbs should be forced if they've not been planted in the gorund by now. See: Forcing Bulb For Indoor Beauty in Winter http://www.extension.umn.edu/distribution/horticulture/DG1116.html Tender roses should be winter protected now, if they've not already been covered. Tipping roses is the recommended method. For info see: http://www.extension.umn.edu/projects/yardandgarden/YGLNews/YGLN-Sept1502.html#roses

Please also see last November's tips at:
http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Nov0103.html#tips

Editorial Notes

Blueberry fall color. Photo credit: Beth Jarvis

Most Minnesota gardeners are probably looking forward to a bit of a break by now. It won't be long before seed orders are sent in and fresh seed packs fill garden center racks and it starts all over.

I spent a week in London in October. I was amazed at how many flowers were still in bloom--flowers I associate with mid-summer blooming here. And the roses were still looking good, also. But the Kew gardeners were busy pulling annuals and the Hampton Court gardeners had cleaned the formal garden of annuals as well.

For Dec. 1, Patrick Weicherding, Regional Extension Educator, Forestry, will be writing about new research findings on the timing of pruning trees for disease prevention and to prevent pruning damage. Tom Haubach, our composting guru was so entranced with the changes he noted in his home compost pile that he's going to share his ruminations a bit later this winter.

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 Thursday 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


Beth Jarvis
Yard & Garden Line Project Coordinator


Previous Page
University of Minnesota Extension Service Home Page
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request.  Please contact your University of Minnesota county extension office or, outside of Minnesota, contact the Distribution Center at (612) 625-8173.

The University of Minnesota Extension Service is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.