|Yard & Garden Line News
Volume 6 Number 8 June 1, 2004
Climate Dynamics and Plant Hardiness Zones
Mark Seeley, Extension Meteorologist and Climatologist
All too often, people forget how dynamic the behavior of climate can be, even here in Minnesota. The variability of climate from year to year and season to season can be relatively small or large, affecting the frequency and magnitude of extremes, such as minimum temperature. Climate statistics derived over a period of time (commonly 30 years or more) give a picture of its general behavior, but the occurrence of extremes can be very random. The most recent example concerns the recent trend in warmer than normal winters. The winters of 1998, 1999, 2000, and 2002 were among the warmest in Minnesota history, yet just preceding those, the winter of 1996 brought all-time record low temperatures to scores of Minnesota communities, including Tower's state record low of -60 degrees F, and the Twin Cities near-record low of -32 degrees F.
| USDA Zones
Whether for commercial or residential landscaping, gardening, or farming, one of the dilemmas faced by Minnesota citizens is the selection of proper plant material to match the climate zone in which they reside. The USDA Plant Hardiness Zones shown in Figure 1 are based on the average annual minimum temperature records. The current zone system has been in use since 1990 and is based on the temperature records for the period from 1974 to 1986, a shamefully short thirteen year climate snapshot that may not necessarily be a good representation of the spatial and temporal variation in minimum temperature. In Minnesota, the Plant Hardiness Zones range from 2b (-40 to -45 degrees F annually) to 4b (-20 to -25 degrees F), with a primarily north to south orientation.
Comparing the 1990 USDA zone map to a similar map shown in Figure 2 and derived by the Minnesota State Climatology Office (SCO) using a larger number of climate stations presents some interesting contrasts. The SCO map shows the urban heat island of the Twin Cities metro area, designated as a 4b zone. In addition, it shows the cold pockets associated with the Jordan climate station in southwestern Scott County and the Zumbrota climate station in Goodhue County, both designated as zone 3b. The very cold pocket in northern St Louis County associated with the Tower and Embarrass climate stations shows a zone designation of 2b in the SCO map, while the 2b zone designation showing in Beltrami County on the USDA map does not appear.
| State Climatology
| SCO 40 year map
The flaw in both of these maps is the short period of record (1974-1986) which may be skewed by the absence or presence of real temperature extremes. A more statistically robust analysis was made by the SCO from the more recent forty-four year record period of 1960-2003, and is shown in Figure 3. This map shows a similar range in zones, 2b to 4b, but depicted with a more uniform and smoother pattern. Zone 4b encompasses most of the southern third of the state, with cold pockets evident in Goodhue, Scott, Pipestone, and Fillmore Counties. Zone 4a can be found stretched across the central portions of the state, except for a small area in Wilkin County. Zones 4a and 4b also form a very tight gradient along the north shore of Lake Superior, a winter landscape greatly modified by the presence of the largest of the Great Lakes. The northern half of the state is dominated by zones 3a and 3b, except for a small fraction of northern St Louis County where zone 2b is evident. From a conservative or risk-adverse perspective this forty-four year analysis is probably a good choice in making decisions about plant materials that are intended to last many years in the landscape. Capturing 44 years of minimum temperatures in such an analysis is a better match to the expected longevity of the most hardy perennial plant material, including many trees.
Finally, a brief look at trends in average annual temperature may be worth noting for those citizens who want to be risk takers in their selection of plant materials. Figure 4 shows the 50-year trend in annual minimum temperature across the state. Only the northeastern section of the state, centered on the Iron Range, shows a negative trend in mean annual minimum temperature. The vast majority of the Minnesota landscape shows an upward trend, in some cases by 6 to 10 degrees F, large enough to shift the Plant Hardiness Zone by one category. Thus those risk takers who want to explore the use of new plant materials in the USDA Plant Hardiness Zone 4a for example, might consider planting some zone 4b species. Though some of these trends may be related to urbanization or changes in land use and drainage, it is difficult to say if the trends will be long lasting. It is interesting to note however, that the most significant temperature changes of the past 50 years in Minnesota are associated with the winter season, much more so than any other season of the year.
| Temperature trends
Iris Leaf Spot
Janna Beckerman, Extension Plant Pathologist
Irises are absolute stars of the garden and provide spectacular blooms with very little in return, except maybe protection from a few disease problems. Iris leaf spot is a common disease caused by the fungus, Mycosphaerella macrospora (formerly Didymellina macrospora). Unusually wet weather, like much of the state has had in the last few weeks, creates severe outbreaks of this disease. Spores produced in the spring are rain splashed onto newly emerging iris leaves, where they begin the infection process. Leaf wetness, coupled with high humidity creates optimal conditions for infection to occur. Overcrowded plants that need dividing develop the disease earlier in the season, and often with greater severity.
| 'Batik' iris.
| Iris leaf spot infection begins.
| Enlarged lesions coalesce, causing severe foliar damage.
Symptoms first develop as small brown spots with water-soaked margins. Although not conspicuous at first, soon after blooming, the spots quickly enlarge and coalesce. This can cause death of the leaf from the tip back to the rhizome. Enlarging spots tend to become oval-shaped and lose their water-soaked margin. As the lesions age, these spots turn a yellow to reddish-brown with gray centers. Premature killing of the leaves can weaken the bulb or rhizome causing the gradual death of the entire plant. Although infection is usually confined to the leaves, stems, flower stalks, and buds may become infected during severe outbreaks of this disease.
Plant irises in full sun, and space plants at least eight inches apart. Providing iris with adequate spacing improves air circulation around leaves, allowing them to dry more quickly, preventing disease development. As this pathogen overwinters in dead iris leaves infected the previous year, removing and destroying diseased leaves in the fall will reduce the amount of fungal inoculum available for infection the following spring.
Frequently, good sanitation and plant health care practices will provide sufficient disease control. However, under severe infections, a fungicide spray program is recommended for the following spring to prevent the disease cycle from repeating. Recommended fungicides include those containing chlorothalonil (e.g. Daconil 2787), mancozeb (e.g. Fore or Dithane), or and thiophanate-methyl (Cleary's 3336). Regardless of which fungicide you choose, start when the leaves are 4 to 6 inches high and repeat at 7 to 10 day intervals, according to label guidelines. When applying fungicide, make sure the spray is applied as a very fine mist to encourage adherence of the fungicide to the leaf surface. If a coarse spray is used, the waxy nature of iris leaves coupled with the parallel venation, may cause the fungicide to run off the foliage. If you are having problems keeping the fungicide on the foliage, add either a commercial spreader-sticker or 1/4 teaspoon of a household liquid detergent to each gallon of spray to aid in wetting the foliage, and adherence of the fungicide.
For more information on iris care and culture, go to the American Iris Society website at:
Is Moss a Problem?
Deborah Brown, Extension Horticulturist
This spring's protracted spell of cool, wet weather has been good for most lawns. It's been good for moss in some locations, too. A lot of people are concerned because they don't like to see any moss in their lawns, but how do you deal with it?
| An extremely mossy lawn.
We tend to associate moss with soil that's too acidic. Perhaps this comes from books and articles written by authors based in the eastern part of the United States, where soils are older, and indeed, more acidic. But in Minnesota, particularly the Twin Cities area, soil acidity is rarely a problem. Besides, Kentucky bluegrass, the major component of most of our lawns, flourishes in a wide range of soils, from those that are relatively acidic to slightly alkaline. A combination of shade and soil compaction are far more likely to be responsible for moss in lawns.
None of our lawn grasses, even those most tolerant of shady conditions, can grow vigorously in heavy shade. They need several hours of good sunlight daily in order to thrive; the shadier the location, the weaker their growth. As they thin, moss – which is perfectly happy in dense shade – can easily fill in and take over, especially when conditions are moist. If shade is a result of large trees, having them pruned or thinned to let more light through may have a beneficial effect on the lawn. But if shade is caused by a nearby building, there's little you can do.
Grass not only needs enough sunlight to grow well, it should have soil that drains well in order to develop a deep, healthy root system. Moss, on the other hand, has shallow little rootlets, and grows just fine on poorly-drained, compact soil. In fact, when soil drains poorly, moisture tends to stay near the surface, right where it does moss the most good.
If soil is hard and compacted, plan to core aerify the lawn regularly to improve drainage. Early autumn works best, because the small plugs of soil unearthed will not result in much weed seed sprouting that late in the season. Core aeration this time of year should be followed up with an application of crabgrass preventer to stop those weed seeds.
Moss may be an indication that the soil is nutrient-poor as well as compacted. Check the U's Soil Testing Laboratory's website http://soiltest.coafes.umn.edu/ to investigate soil testing, then fertilize the lawn accordingly. There are products in nurseries and garden centers that will kill moss in lawns, but to effect a more permanent solution, conditions favoring moss growth must be addressed, or it will just come back.
Add Daisies to Your Garden
Deborah Brown, Extension Horticulturist
Everyone loves daisies. Why not add some to your garden this year? There are dozens to choose from. Several are hardy perennials; others, "true annuals" or "tender perennials" that won't survive our cold winters. All add simplicity and old-fashioned charm to gardens and landscapes.
We usually use the term "daisy" to describe a composite flower made of one or two rows of outer petals called ray florets, and a cushion-like center composed of disk florets. Each daisy is actually an aggregate of many tiny flowers. We think of "classic" daisies as white with gold centers, but there are equally lovely daisies and daisy lookalikes in shades of lilac, purple, pink and rose, yellow, gold, and orange, mahogany, and combinations of colors.
The earliest daisy, leopard's bane,(Doronicum cordatum), should be planted more frequently in Minnesota. It's clear yellow blossoms are quite eye-catching, blooming alongside tulips, allium, and other late flowering spring bulbs.
| Leopard's bane.
| Oxeye daisies.
Leopard's bane grows somewhere between 1 and 2 1/2 feet tall, producing 2 to 3 inch flowers, depending on which cultivar you choose. These plants thrive in soil containing lots of peat or other organic matter, but do not require full sunlight. Light shade is ideal.
Oxeye daisies, Leucanthemum vulgare (also called field daisies), look like a more delicate version of Shasta daisies, with flowers only an inch or two in diameter. They spread vigorously by means of underground rhizomes and are considered an invasive species by the DNR because they compete with native plants when they get loose in the countryside. Even though they're pretty, it's not a good idea to plant them.
Thread-leaf coreopsis or tickseed includes a number of daisy-like cultivars that are steadily gaining in popularity. 'Moonbeam,' a plant that forms mounds of dainty light yellow daisies begins blooming in late spring, but unlike most perennials, continues to bloom throughout the summer. 'Zagreb' (yellow-gold flowers) begins a bit later than 'Moonbeam,' but is more reliably winter-hardy.
Coreopsis rosea sports rose-colored blossoms, unique among tickseeds. It is also a little more shade-tolerant than the others, and can take more moisture. Most coreopsis species will perform best given a location with light, well-drained soil and full sun. Newer, red-flowered cultivars have not proven reliably hardy in most of Minnesota.
| Pink coreopsis
Painted daisies, Tanacetum coccineum (may still be listed as Chrysanthemum coccineum) are prized for their red, pink, or rose- colored flowers borne on tall, wiry stems. They bloom in early summer, growing best under conditions similar to the tickseeds; full sun and open, well-drained soil. Painted daisies make excellent cut flowers; they're also the source of commercially produced Pyrethrin, a rather toxic organic insecticide.
Blooming from mid through late summer, Shasta daisies (Leucanthemum superbum) are truly the royalty of the daisy world. Unfortunately they're not quite as cold hardy as we'd like. 'Alaska' and 'Silver Princess' are among the hardiest cultivars, but even they benefit from a thick layer of winter protection applied each fall after the ground has frozen.
| 'Snow Lady' Shastas
Plant shasta daisies in full sun or light shade in rich, well-drained soil. Too much shade will reduce flowering and make stems floppy. Fertilize these plants lightly but regularly throughout the summer. Besides looking great in the garden, they'll provide beautiful, long-lasting cut flowers.
Later in the season you can count on hardy mums, Dendranthema x rubella, to produce long-lasting cut flowers with several buds on each stem. Of these, the old-fashioned 'Clara Curtis,' a rose pink daisy with a yellow center is best known and most commonly grown in Minnesota gardens. 'Mary Stoker,' a yellow-beige cultivar, is also lovely.
Don't confuse these with typical garden mums -- they are more reliably hardy and can be expected to come back year after year. For best results, though, plant them in full sunlight. Blooming will be reduced in partial shade and growth will be spindlier and more prone to falling over in heavy rain.
The more robust series of mums marketed as "My Favorite" include several attractive daisies in different colors, including a deep red one called My Favorite ‘Autumn Red.' These are the mums that become incredibly enormous cushions covered by thousands of blossoms after two years in the garden. They're perfectly hardy in the Twin Cities area, and are highly recommended.
| 'Autumn Red'--a My Favorite Mum
Some Annual Daisies worth noting:
Many of the best annual daisies are suited to bright, hot, sunny locations which makes them ideal for our summer gardens. While you'll need to water them periodically if weather turns exceedingly dry, these "daisies" are generally quite drought tolerant.
Calliopsis produces masses of small flowers in combinations of yellow, gold, dark red, and mahogany. They're held on thin, wiry stems from 1 to 3 feet tall, depending on cultivar. Taller stems are often cut for bouquets. Once established, calliopsis tends to self-seed.
Gazania flowers are both large (up to five inches across) and strikingly colored with bands of contrasting colors in concentric circles out from their centers. Most are in the yellow-gold-orange range, with green, black, or various shades of brown and bronze towards the center.
Gerbera daisies come in a wide variety of colors, from pastels to deepest crimson. They produce huge, long-lasting blooms, but usually only a few at a time. These important-looking plants deserve a place in a large container where they can be easily appreciated. They're usually considered too costly to mass in the garden.
African daisies thrive as annuals in sunny, well-drained locations here. Osteospermum species include a large array of tropical colors that will pep up any garden setting. They're a far cry from pristine white Shasta daisies, but very lovely none-the-less. Osteospermum in the "Symphony" series are more heat tolerant than others, and should bloom consistently throughout the summer here.
Whitemarked Tussock Moths
Jeff Hahn, Assistant Extension Entomologist
Whitemarked tussock moths are common insects, belonging to the family Lymantriidae (tussock moths and their relatives). This group of moths also includes the infamous gypsy moth. Whitemarked tussock moths feed on many deciduous trees and shrubs, including linden, apple, birch, elm, poplar, and dogwood.
| Whitemarked tussock moth
This caterpillar is very distinctive. It has a reddish head and a hairy yellowish body. Also look for two long black tufts of hair that stick out near the head, a yellowish or blackish tuft at the tip of the abdomen and four cream colored squarish tufts on the top of its body (it kind of looks like toothbrush bristles). Whitemarked tussock moth caterpillars are about 1¼ inch long when full grown.
They spend the winter as eggs and hatch the following spring between mid to late May. The young larvae windowpane feed on the leaves (this is sometimes referred to as skeletonizing). This type of injury is when insects feed on one layer of leaf tissue between the veins, giving leaves a lacey or transparent look. As the caterpillars become older, they consume the entire leaf, except for the petioles, midveins and other large veins. The caterpillars feed for about five to six weeks. There is a second generation that hatches in August and feeds into September.
In most cases whitemarked tussock moth numbers are not abundant enough to seriously defoliate woody plants. And most of the time they are attacking healthy, well-established trees and shrubs which can easily tolerate their feeding even if it is severe. The general rule of thumb is that trees and shrubs that are healthy and well-established can tolerate severe feeding, even complete defoliation, for three consecutive years. Although such feeding probably would slow down their growth, plants would not be killed.
However, if you have woody plants that have been recently transplanted (within the last couple of years) or are stressed and unhealthy, even moderate numbers of whitemarked tussock moth caterpillars can be damaging. The extra stress of insect feeding while trees and shrubs are more vulnerable can cause branch dieback and could eventually kill woody plants.
If it is necessary to manage whitemarked tussock moth caterpillars, the key is to treat them when they are small, when they are 5/8 inch long or less. If the insects are full grown or nearly so, it's too late for control. You want to minimize plant damage not just kill insects. Spraying caterpillars not only does not reduce the number of insects you'll see later but probably will unnecessarily kill natural enemies. So only treat caterpillars when their management results in reducing their damage to plants.
Less toxic insecticides include azadirachtin (Neem), Bacillus thuringiensis (Dipel, Thuricide), and spinosad (Conserve). Other effective insecticides include acephate, carbaryl, cyfluthrin, and permethrin.
Ants in Turf
Jeff Hahn, Assistant Extension Entomologist
Ants are very numerous insects and are particularly common in lawns. Cornfield ants, field ants, pavement ants are common species that can be found nesting in yards. Ants impact your lawns in several ways. First they are important in the environment as predators on other insects and invertebrates found in turf and nearby areas. Soil-nesting ants often feed on honeydew secreted by aphids that they find on plants. This can have the indirect action of protecting aphids as they feed.
| Cornfield ant nests
The nests themselves can create bumps in lawns. Depending on the species of ants, the mounds can be relatively small, not much more than an inch or two wide as is the case with cornfield ants or they can be considerably larger like field ants that can have mounds that are a foot or more in diameter. Larger mounds can also smother nearby grass. Generally when you see ants mounds in bare areas of lawn, the ants did not kill that grass but instead are taking advantage of pre-existing problems in the lawn.
We are fortunate not to have fire ants in Minnesota and so the risk of bites and stings are minimal. However, it is possible to be ‘attacked' by certain ants, e.g. field ants which are moderate sized (1/4 inch long) can bite when defending their nest. Pavements ants (1/8th inch long) are also known to bite and sting if their nest is threatened. These ants would generally not be aggressive towards people otherwise. Also ants that nest in lawns close to buildings can potentially enter structures to forage for food and water or to nest.
| Field ant nest
Management of ants in turf is difficult and at best just temporary. If nests are treated in an area, new nests will re-colonize that site after the insecticide residue dissipates. It is never suggested to attempt to treat all or most ant nests in a yard as you can never stay ahead of them. It would become an unending circle of pesticide treatments which is not practical. At best, you can target one or a small number of nests that are particularly being a nuisance or is implicated with problems indoors.
There are variety of insecticides you can use to treat ant nests, such as permethrin (as a liquid or granules), carbaryl (as a liquid or granules), bifenthrin (as granules), cyfluthrin (as granules), or (acephate as a liquid). Be sure to select a product that is labeled for treating lawns.
Get the low down on this month's insect pests at Insects http://www.extension.umn.edu/projects/yardandgarden/EntWeb/Ent.htm
Early June Garden Tips
Beth Jarvis, Yard & Garden Line
Compiled from conversations with David Hanson, Urban Forestry; Bob Mugaas and Bob Olson, Regional Extension Educators
| Blooming blueberries.
Drought Update for June 1
Mark Seeley, Extension Climatologist, says the first half of June should be cooler than normal but the rainfall should return to normal amounts.
(These recommendation are based on Twin Cities temperatures. Adjust for northern Minnesota..)
Trees and Shrubs:
To reduce birch leaf minor damage keep birch roots cool and shaded. Use a four inch depth of woodchip mulch under the canopy of the tree.
Choose a dry day to prune trees. Spores of some tree diseases spread during humid or damp summer weather.
Water Trees! Trees need at least one inch of rainfall each week May through July and a little less in August and September.
Prune trees for clearance over streets and sidewalks and near signs. Many cities require 13 feet of clearance over streets and 8 feet over sidewalks.
Look up before you plant. What will the mature tree be growing up into? Consider the ultimate spread of the tree in relationship to not only power lines but buildings. Remember too that the roots can, in sandy soil, spread as much as three times the canopy.
Lastly, when spraying lawn weeds, remember that trees are broad leafed plants and can be injured by herbicide, also. Pay particular attention to where you apply herbicides containing dicamba–such as Trimec. Dicamba is absorbed by tree roots, unlike 2,4-D.
With the cool spring we've had of late, cool lawns might still get the benefit of some pre-emergent for crabgrass.
If you see the crabgrass coming up, you can spray it w/post emergent crabgrass control. Spraying annual grasses, like crabgrass is tough when the desired grasses are stressed (as in mid-summer), but now would be a fine time.
Lawns on sandy soils might be looking a little faded, washed out after all the rain. It's ok to give them a little extra N, at 1/3 to 1/2 N/1,000 sq. ft.
You can still spray deandelions and other already emerged broadleaved weeds.
Start raising your mower height up heading toward 2.5-3" high for summer. The taller grass not only shades the roots, preserving soil moisture but the roots grow deeper when the tops are longer.
It's ok to let your grass get a bit dry. We say grass needs 1 to 1.5" of water per week, but a touch of drought stress encourages deeper, more extensive rooting.
Root crops like carrots, radishes, beets, turnips, and parsnip should be thinned to approximately 2 inches between plants within the row.
Don't harvest asparagus spears until plants are at least 3 years old; otherwise you'll be sacrificing long term survival. When harvesting spears, snap them off at the soil line or use a knife to cut just below the surface.
Remove leaves from vegetable plants that appear water-soaked or otherwise diseased to reduce further spread of disease organisms. These leaves are generally lower to the ground and are susceptible to soil plashing.
Begin hilling potatoes by mounding soil around the base. This forces stem elongation and increases tuber formation.
Always maintain soil coverage over the forming tubers; otherwise sunlight will result in greenish potatoes that are bitter tasting.
Despite the wet weather, now is a good time to plan an irrigation system for the vegetable garden. Trickle irrigation systems are highly efficient, labor saving, and result in less disease pressure compared to overhead sprinkling. There are a variety of home garden trickle technologies that can be employed. Here are a couple links to sites that provide some novel ideas that Master Gardeners might want to pursue:
Apply nitrogen to new strawberry and raspberry plantings. Apply about 1 pound of ammonium nitrate per 100 feet of row, or about ¾ pound of urea per 100 feet of row.
Early June is an ideal time to train young fruit trees. Training goes hand-in-hand with pruning; however, pruning should no longer be attempted this growing season. Training is the physical manipulation of the branches such that the trees form stronger crotch angles and yield more fruit. The ideal crotch angle for trees is approximately 60 degrees from the trunk. To achieve this, you can use a variety of methods to force these angles. SPREADER devices can be wedged into the branches (these can be clothes pins, short sticks, or other spacers); branches can be TIED to adjacent branches using twine, strips of cloth, and even large rubber bands on small trees; and branches can be affixed with WEIGHTS that attach by clothes pins or cloth.
Young fruit trees should be staked soon after planting. Unstaked trees are easily bent by winds and develop poor growth habits. Drive the stake about 4 inches away from the stem, and attach the tree to the post with tape or soft cloth. Do not use wire or other materials that will prevent the trunk from expanding. Even tape and cloth attachments should be periodically checked for constrictions. Rapidly growing trees are very weak; some fruit trees will require staking for up to 5 years before they have developed sturdy wood.
Don't braid or cut foliage of sping flowering bulbs. Remove the foliage once it has has yellowed, if you like.
Deadheading (removing spent flowers) from annuals will keep them blooming all summer.
Squirrels are chewing the tips of elm branches leaving sidewalks covered with twig ends.
As of June 1, the Twin Cities has had 11 straight days of rain. We are definitely recovering from last year's late summer drought but at the expense of getting gardens planted. Pulling rampantly growing weeds is easy, though, with the ground as wet as it is.
| Virginia bluebells.
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