Ornamental Horticulture Monthly Newsletter
Volume 2 No. 6, October 1999
Over-wintering
Herbaceous Perennials
Robert Berghage and Jim Sellmer
The decision making process for over-wintering container nursery stock
begins by matching plants to the proper over-wintering system and cultural
practices to assure survival and rapid spring growth. Whether we are talking
about woody or herbaceous landscape plants the ultimate reason for providing
winter protection for container stock is to protect the root system from
cold temperature and drastic temperature fluctuations. Common cultural
practices involved in over-wintering container nursery stock include fertility
before and during over-wintering, irrigation, pest control, foliage removal,
and the timing of covering and uncovering.
Fertilization
The widely held belief that late-season fertilizer applications decrease
plant cold hardiness has not been supported by research (Pellett and Carter,
1981). The general rule of thumb is that healthy plants that are not excessively
vigorous are better able to withstand winter freezes and thus fertilizer
applications should provide enough nutrients to sustain only normal growth
and development. Vegetative growth slows in late summer-early fall which
marks the time to gradually reduce the rate of nitrogen applied to approximately
half of the rate applied during the growing season (Beattie, 1986). Late
August to late September is the window for completion of reduced N fertilization
for Hardiness zones 5 &6 and 7 & 8 respectively. Seasonal change in temperatures
also affects the choice of N fertilizers for use in container nursery
stock. Nitrate nitrogen forms of fertilizer are more available when the
medium is cool to cold. While the soil remains warm, the roots of perennials
continue growing and under this condition, the nitrate form of fertilizer
can benefit root development.
Irrigation
Two issues come into play with moisture management in over-wintering.
Research shows that water stress prior to over-wintering can enhance cold
hardiness of many plants during the hardening off process (Levitt, 1980).
However, irrigating to field capacity prior to storage provides an essential
source of moisture along with a sufficient moisture or humidity level
within the storage structure during the winter to prevent freeze drying
of the plants and soil. The necessity and ability to mist or irrigate
during the winter is dependent on the over-wintering system employed.
Pest Control
Pest control includes rodents, disease, and weed management. Rodents can
be a significant source of damage to both open and closed winter storage
systems. Rodent damage varies from girdled roots and stems to crown damage
to complete consumption of the plant. Rodents can be controlled by use
of baits, traps, and predators such as cats.
Winter annuals and perennial weeds are a particular problem within in
closed over-wintering structures (Beattie, 1986). Few herbicides are labeled
for use in closed structures. For this reason, a progressive program of
preparing the over-wintering structure with nonselective herbicides like
Roundup combined with a preemergence herbicide product 2-6 weeks prior
to closing the structure will help in controlling weeds and reducing weed
contamination of containers next spring. During preparation for winter
storage the containers should be weeded and treated with a preemergence
herbicide at least 2-6 weeks prior to storage. The timing and preemergence
rates are dependent on products chosen and all labels and precautions
should be followed. To assure good weed control a similar weeding and
preemergence treatment process using a different herbicide should be applied
to the containers after they are uncovered in the spring (Kuhns, 1997).
The Penn State Extension publication "Controlling Landscape and Nursery
Weeds provides a listing and description of appropriate herbicide products
and is available from your county cooperative extension office.
Fungi thrive in warm, moist conditions that are common in the autumn,
late winter, and early spring when plants are under thermal blankets and
plastic coverings. Foliage removal prior to storage, fungicide treatments,
and ventilation during warm periods can reduce the prevalence of fungal
diseases. Remember to read and follow all pesticide labels and precautions
before applying any pesticide.
Over-wintering Systems
In any over-wintering system, a series of microenvironment changes occur
that impact the plants being over-wintered. Your choice of systems creates
a specific set of management requirements. Choosing a system that relies
on a cover provides the greatest protection against cold temperatures
and extreme temperature fluctuations. It also results in heat buildup
during transition periods and unexpected warm-ups. To avoid this, the
recommendation is to cover plants at the latest possible moment in the
fall after hard frosts have triggered leaf drop but before prolonged freezing
has begun. Covering too early can delay hardening off of plants, result
in overheating, and premature growth of tender new shoots. Conversely,
the timing of uncovering is just as important. Remove coverings prior
to overheating, excessive bud swell, and after subfreezing temperatures
have passed. Timing, instinct, and flexibility in the over-wintering structure
that you use are important in successfully managing over-wintering plants.
Characteristics to consider in choosing an over-wintering system include:
Does your system provide adequate protection
to plant roots?
Does it reduce moisture loss thus avoids tissue
burn?
Is it easily handled and managed?
Is it economical?
Can it be adapted to the growth habit of the
plant?
In most nursery and garden center situations, more than one system is
employed to meet the above needs. Generally, the systems used in Pennsylvania
include the use of coverings within a structure such as a polyhouse, or
in conjunction with a structureless system. The best coverings include
a white reflective plastic, foam, or cloth to reduce the build up of heat
in the system due to the sun. Single layer films are the most common covering,
such as a 4 mil white copolymer (white poly). Within structures, a clear
polyethylene or a white copolymer may be used alone or in combination
with other insulating materials. These materials may be suspended slightly
above the plants or placed directly over the plants.
Over-wintering systems commonly used by the industry are listed below:
Structureless systems - no frame structure used
Plants consolidated with no added covering or
wrapping
Plants consolidated and surrounded with one
layer of kraft paper
Plants consolidated and surrounded with bales
of straw
Plants consolidated and covered with one layer
of white polyethylene film
Plants consolidated and covered with a thermal
blanket (microfoam; polyethylene bonded to microfoam)
Plants consolidated and covered with a thermal
blanket and polyethylene
Polyhuts - low, white polyethylene-covered structure (4 ft high)
Plants consolidated inside polyhut with no
added covering
Plants covered with one layer of white polyethylene
Plants covered with a thermal blanket
Plants consolidated and covered with a thermal
blanket and polyethylene
Polyhouses - taller and wider polyethylene-covered structure (7 ft
high)
Plants consolidated inside polyhouse with no
added cover
Plants covered with one layer of white polyethylene
Plants covered with a thermal blanket
Plants covered with a thermal blanket and a
single layer of white poly
Polyhouses with inflated double polyethylene cover
Plants consolidated inside polyhouse with no
added cover
Plants covered with one layer of white polyethylene
Plants covered with a thermal blanket
Plants covered with a thermal blanket and one
layer of polyethylene
Plants not covered, but polyhouse is heated
Research on over-wintering systems and techniques continues to provide
additional options and methods for growers to try and adopt (Iles et
al 1993, Perry 1990, Still et al 1989). However, developing new techniques
for over-wintering container landscape plants may not be effective if
we do not know what growers, landscapers, and garden centers are using
to shelter their plants from winter injury. Jeff Iles of Iowa State
recently reported on a survey study of the US and Canada to identify
and gauge the effectiveness of winter protection systems in zones 3-8
(Iles, 1999). The following table presents the results from both a nationwide
survey regarding zones 4-7 that best represent the zones of PA and a
local survey conducted in western PA during a conference on production
of herbaceous perennials.
The following table presents the results from both a nationwide survey
regarding zones 4-7 that best represent the zones of PA and a local survey
conducted in western PA during a conference on production of herbaceous
perennials.
|
Nationwide
|
Western PA
|
General Information
|
|
|
| Response rate |
46% 293 Firms
38 states & 6 provinces |
26% 34 responses |
|
| Firm |
% responses |
% response |
| Retail nursery/garden center |
41%
|
68%
|
| Landscape design/construction |
8%
|
18%
|
| Rewholesale nursery |
2%
|
6%
|
| Production nursery |
45%
|
23%
|
| Other: Greenhouse, tree care, public gardens |
5%
|
9%
|
|
|
|
| Acclimating for storage |
|
|
| Do not apply fertilizers after Sept 15th |
68%
|
65%
|
| Reduced irrigation to assist hardening-off |
46%
|
59%
|
|
|
|
| Cultural practices at covering |
|
|
| Irrigate |
55%
|
62%
|
| Fertilize |
6%
|
21%
|
| Fungicide |
33%
|
29%
|
| Rodenticide |
59%
|
41%
|
| Remove foliage |
73%
|
47%
|
| Other: Insecticide, Re-pot |
6%
|
6%
|
|
|
|
| Over-wintering systems |
|
|
| Structureless |
71%
|
65%
|
| Polyhut |
12%
|
21%
|
| Polyhouse |
53%
|
56%
|
| Polyhouse with inflated double cover |
31%
|
21%
|
|
|
|
| Coverings & other methods |
|
|
| Respondents applying no covering |
--
|
76%
|
| Applying white polyethylene |
--
|
32%
|
| Applying thermal blankets |
--
|
15%
|
| Double layer covers |
--
|
8%
|
| Polyhouses with minimum heat |
--
|
15%
|
| Plant loss & winter management |
|
|
| Does your system permit monitoring? |
--
|
82% yes
|
|
|
|
| My over-wontering system results in: |
|
|
| No Plant Loss |
--
|
6%
|
| 1-10% plant loss |
81%
|
53%
|
| 11-25% plant loss |
19%
|
26%
|
| 26-50% plant loss |
--
|
9%
|
| Greater than 50% plant loss |
--
|
3%
|
|
|
|
| Factors most responsible for plant loss |
|
|
| Insufficient low temperature protection |
33%
|
18%
|
| Too wet |
50%
|
44%
|
| Too dry |
15%
|
15%
|
| Over-wintering protection applied too late |
8%
|
3%
|
| Over-wintering protection removed too late |
7%
|
9%
|
| Animal damage |
33%
|
47%
|
| Disease |
20%
|
18%
|
| Other |
16%
|
9%
|
|
Temperature fluctuations
|
|
|
|
Winter covering pulled too early in spring
|
|
|
The final question asked to the respondents in Pennsylvania was to list
their top five most difficult plants to over-winter in containers. The
following list reports the most difficult based on votes (the value to
the right of the name) and the others considered difficult by individual
respondents.
Difficult to Over-winter Perennials
|
| Ornamental grasses (+6) |
Hedge-Like Shrubs |
| Dephinium spp. Delphiniums (+5)* |
Hesperis matronalis Sweet Rocket |
| Geum (+2) |
Hemerocallis spp. Daylily |
| Heuchera spp. Coalbells(+2) |
Iris spp. Iris |
| Dianthus spp. Pinks (+1)* |
Linum spp. Flax |
| Digitalis spp. Foxgleove (+1) |
Lupine spp. Lupine* |
| Guara (+1) |
Lychnia chalcedonica Maltese Cross |
| Hosta spp. Hosta (+1) |
Monarda didyma Bee Balm |
| Phlox subulata Creeping Phlox (+1)* |
Nepeta siberica Catmint |
| Aquilegia flaellata Columbine |
Oenothera speciosa 'Siskiyou' and 'Rosea' |
| Asclepias tuberosa Milkweed* |
Oenothera tetragonaEvening Primrose |
| Aster spp. Aster |
Phlox paniculataTall Phlox* |
| Chrysanthemum spp. Mums |
Platycodon grandiflorusBalloon Flower |
| Coreopsis Rosea Pink Coreopsis * |
Primula spp. Primrose |
| Ferns |
Pulmonaria spp. Lungwort |
| Gaillardia x grandiflora Blanletflower |
Salvia spp. Sage |
| Scabiosa spp. Scabiosa/Pincushion flower |
| Sedum spp.Stonecrop |
|
| Verbena spp. Verbena |
Plants marked with an asterisk (*) was also listed among the top 10 diffucult
plants on the nationwide survey.
Observations from the survey data
Garden Centers and retail nurseries are the
majority of firms over-wintering herbaceous perennial stock in Pennsylvania
based on the respondents. Whereas nationwide, retailers and growers were
the vast majority of the over-wintering firms.
Fertilization practices stopped by Sept. 15
in both the nationwide survey and locally. The practice of avoiding late-season
fertilizer treatments continue even though research does not support the
perception that late season fertilizer applications decrease plant hardiness
(Pellet and Carter, 1981).
A majority of the Pennsylvania respondents reported
reducing irrigation in the fall. Water stress because of reduced watering
has been reported to enhance cold hardiness (Levitt, 1980).
The Pennsylvania respondents reported a greater
likelihood to fertilize prior to covering plants compared to the nationwide
respondents. Conversely, they were less likely to employ a rodenticide
or remove foliage than the nationwide survey respondents were.
Polyhut over-wintering structures remain popular
among Pennsylvania respondents.
Greater than 75% of the Pennsylvania respondents
rely stictly on the coverings standard with their system and do not employ
additional coverings. In addition, where added coverings are employed
white poly is used more often than thermal blankets. Few respondents were
over-wintering in polyhouses with heat.
Greater than half of the Pennsylvania respondents
reported zero to minimal losses during over-wintering under their chosen
systems.
The greatest reasons for plant loss during over-wintering
in Pennsylvania was attributed to animal damage and excessive moisture
within winter protection systems. The high level of animal damage may
be directly attributed to the limited use of rodenticide or other animal
control options.
Over half of the respondents of the Pennsylvania
survey reported using structureless systems where ventilation is often
poor. Participants, expressed an interest in reducing the need for watering
during the over-wintering period. For those that are irrigating under
inclimate conditions and manual watering may be lending to the excess
moisture within the structure. The difficulty in managing moisture within
the over-wintering structure suggests that the management system needs
to be better defined. Several research options include the use of hydrogel
dips or as a soil coverings and the implementation of automated watering
and humidity management systems.
Successful over-wintering of herbaceous perennials
remains as much an art as a science. Ornamental grasses, are listed as
one of the most difficult perennials to over-winter. Defining the over-wintering
parameters for ornamental grasses may be as complex as the grasses are
diverse. This diversity is evident between genera, but also among species
for example Miscanthus sinensis varieties that are both visually and culturally
different.
Many techniques and systems which have been defined for over-wintering
woody landscape plants and herbaceous perennials. Although techniques
for over-wintering continue to be created and defined, there remain challenges
and opportunities to define the systems for new ornamentals such as ornamental
grasses and further develop and understand the over-wintering cultural
required to successfully over-winter difficult to store perennials.
Beattie, DJ. 1986. Principles, practices, and comparative costs of overwintering
container-grown landscape plants. South. Coop. Series Bulletin 313.
Iles, JK, NH Agnew, HG Taber, and NE Christians. 1993. Evaluations of
structureless overwintering systems for container-grown herbaceous perennials.
J. Environ. Hort. 11:48-55.
Iles, JK. 1999. Identification and Characterization of Overwintering Methods
for Container-grown Herbaceous Perennials. HortScience Abstract #264.
Kuhns LJ. et al. 1997. Controlling Weeds in Nursery and Landscape Plantings.
Penn State University, College of Agricultural Sciences Publications.
Levitt, J. 1980. Responses of plants to environmental stresses. Vol. 1
Chilling, freezing, and high temperature stresses. Academic press New
York.
Pellett, HM and JV Carter. 1981. Effect of nutritional factors on cold
hardiness of plants. Hort. Rev. 3:144-171.
Perry LP. 1990. Overwintering container-grown herbaceous perennials in
northern regions. J. Environ. Hort 8:135-138.
Still SM, T Disabato-Aust, and T. Rhodus. 1989. Overwintering systems
for herbaceous perennials. Proc. Perennial Plant Symp., pp. 24-35.
Gardener Selects for 2000
The Penn State Master Gardeners, Horticulture Trial Gardens, the Pennsylvania
Floral Industries Association and the Pennsylvania Landscape and Nursery
Association are pleased to announce the Pennsylvania Gardener
Selects. This is a new program for plant evaluation, display,
education, and promotion. Last summer 22 of the best flowering annuals
selected from plants tested in the University trials were displayed
and evaluated in 29 Master Gardener gardens in 26 Pennsylvania counties.
From these 22 plants, the best was named as our first "Plant of
the Year" and 7 others were named Gardener Selects for the year
2000. This is the first year for what we hope will be a long-term plant
selection and promotion campaign. The primary goal of the program is
to identify and promote superior garden plants for Pennsylvania, thus
improving consumer success and satisfaction, and as a consequence industry
profitability.
Gardener Selects Plant of the Year for 2000
Petunia ‘Misty
Lilac Wave’ a Pan
American Seed cultivar
Gardener Selects for 2000
Argyranthemum ‘Butterfly’
Proven Winners
French Marigold ‘Bonanza
Bolero’ Ball Seed
Pepper ‘Super Chili’
Johnny’s Selected Seed
Petunia ‘Pink Wave’
Pan American Seed
Petunia ‘Purple
Wave’ Pan American Seed
Salvia faranacea ‘Signum’
Bodger Seed
Zinnia ‘Profusion
Orange’ Sakata Seed
As a service to their members, the PFIA and PLNA will be participating
in developing promotional materials for these plants. The promotion
will include point of purchase materials, press releases, and display
in the Master Gardener county gardens and at selected industry and
consumer events such as the Philadelphia Flower Show. Plant of the
Year programs in other States has generated great benefits for their
ornamental plant producers and retailers. You can support this program
by growing and marketing these plants. Contact your association for
information on obtaining promotional materials.
The strength of the Pennsylvania program is our use of statewide
Master Gardener display and educational gardens. This expands the
visibility of these superior plants to every corner of the state.
Educational programs and public access to the gardens provides all
interested consumers with the opportunity to see, evaluate, and learn
about these plants. The Penn State Master Gardener involvement provides
a local focus to generate community interest and local publicity.
Last summer several thousand potential customers visited these gardens
during various formal programs, informal visits, or read about the
plants in local newspaper articles describing the program. Involvement
of the Master Gardeners also provides the potential for perhaps the
best endorsement possible for the selected plants; word of mouth recommendations
by respected community leaders. Local display gardens are located
in public parks or at County Extension offices and are open to the
public during daylight hours. Feel free to visit and use these sites
in your marketing efforts.
County
|
Garden Location
|
Phone
|
|
|
County
|
Garden Location
|
Phone
|
Adams
|
West side of Bus Rt 15, Gettysburg |
717-334-6271 |
|
|
Delaware
|
Penn State Delware County Campus |
610-690-2655 |
Beaver
|
911 Center, Beaver |
724-774-3003 |
|
|
Fayette
|
Blaney Farm, Perryopolis |
724-438-0111 |
Berks
|
Berks County Ag Center |
610-378-1327 |
|
|
Franklin
|
919 Franklin Farm Lane, Chambersburg |
717-263-9226 |
Blair
|
PSU Altoona Campus, Altoona |
814-693-3265 |
|
|
Jefferson
|
Heritage House Senior Center, Brookville |
814-849-7361 |
Bradford
|
701 South Fourth St., Towanda |
570-265-2896 |
|
|
Lancaster
|
Farm and Home Center, Lancaster |
717-394-6851 |
Bucks
|
Neshaminy Manor Center, Doylestown |
215-345-3283 |
|
|
|
SE Res. and Extension Ctr., Landisville |
717-921-8803 |
Butler
|
Roebling Park, Saxonburg |
724-287-4761 |
|
|
Lawrence
|
St. Francis Hospital, New Castle |
724-654-8370 |
|
Seven Fields Borough Community Center |
724-287-4761 |
|
|
Lehigh
|
Burnside Plantation, Bethlehem |
610-391-9840 |
|
|
Cranberry township, Municipal Bld. |
724-287-4761 |
|
|
Mercer
|
Mercer County 4H Park |
724-662-3141 |
Cambria
|
County Extension Office, Ebensburg |
814-472-7986 |
|
|
Monroe
|
Monroe Co. Vo-Tech School, Bartonsville |
570-421-6430 |
Carbon
|
Carbon Co. Environmental Ctr. Summit Hill |
717-424-1984 |
|
|
Montgomery
|
Montgomery Extension Ctr Collegeville |
610-489-4315 |
Centre
|
Penn State Trial Gardens, U. Park |
814-862-7725 |
|
|
Northumberland
|
161 Moser Rd, Daville |
570-644-4455 |
Crawford
|
13400 Dunham Rd., Meadville |
814-333-7460 |
|
|
Westmoreland
|
Donohoe Ctr. Greensburg |
724-837-1402 |
Dauphin
|
Milton Hershey School, Hershey |
717-921-8803 |
|
|
Wyoming
|
28 State Rt 108, Tunkhannock |
570-836-3196 |
|
|
|
|
|
|
York
|
Rudy County Park, York |
|
Every year extension offices and agents are inundated with questions
regarding the identification and use of annuals and herbaceous perennials
in the landscape. To assist you in learning more about annuals and herbaceous
perennials and in answering the questions of your clientele, a new web
based resource is available for use within the extension office. Dr.
Dave Beattie is providing for in-office use his interactive course materials
for annuals and perennials. Contact Dr. Beattie at 814-863-2263 or at
b50@psu.edu for a password and instructions for using this valuable
web based resource.
Information provided by Rob
Berghage and
Jim Sellmer Penn State Department of Horticulture, Ornamental Horticulture
Extension Specialist, 103 Tyson Building, University Park, PA 16802, (814)
863-2571
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