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Jeffery K. Iles

Pruning aboveground tissues back to the plant crown in preparation for winter is a common cultural practice for garden chrysanthemums [Dendranthema grandiflorum (Ramat.) Kitamura]. But some landscape managers suggest pruning immediately before the onset of low temperatures may be responsible for predisposing plants to winter injury. To evaluate the effect of pruning garden chrysanthemums in November and December on winter survival, rooted cuttings of 19 chrysanthemum cultivars were obtained from Yoder Brothers and were field-planted in a randomized complete-block design with five replications. Pruning treatments were 1) plants pruned to 2 cm above the crown on 1 Nov., 2) plants pruned to 2 cm above the crown on 1 Dec., and 3) plants not pruned. Survival and regrowth data were gathered the following summer. Cultivars differed in their response to the treatments, but in general, survival percentages and regrowth shoot dry weights were greater for plants that were not pruned.

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Jeffery K. Iles

A survey was conducted to identify and characterize the effectiveness of overwintering methods used to protect container-grown herbaceous perennials in USDA hardiness zones 3 through 8. Survey questionnaires were sent by first-class mail on 20 Aug. 1996 to 634 firms involved in growing and/or selling container-grown herbaceous perennials identified from the Perennial Plant Association Membership Directory. Completed questionnaires were received from 293 individuals (46.2% response rate) in 38 states, the District of Columbia, and six Canadian provinces. Survey participants reported using several overwintering methods: structureless systems (71.0%), polyhouses (52.9%), polyhouses with inflated double polyethylene covers (30.7%), and low-profile polyhuts (12.3%). Over three-fourths of the respondents (78.8%) said their winter protection methods resulted in minimal to no plant loss (0-10%). Only 53 respondents (18.1%) reported losses >10%. The most frequently cited reason for plant loss across all hardiness zones was excessive moisture inside the overwintering environment (50.2%). Equal percentages (33.4%) indicated low temperatures and damage from animals as the next most likely factors responsible for plant loss. Respondents identified, in descending order, Iris, Delphinium, Lavandula, Papaver, and Lupinus as the five genera most difficult to overwinter.

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Jeffery K. Iles

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Jeffery K. Iles

Controlled-freezing tests were conducted in Dec. 1994 and Jan. 1995 to determine cold hardiness of garden mums (Dendranthema grandiflora Tzvelev.) `Baby Tears', `Debonair', `Emily', `Megan', `Ruby Mound', and `Triumph' grown outdoors in central Iowa. Dormant, intact crowns were harvested from the field on 3 Dec. and held at 2 ± 2C until freezing tests began. Crowns were placed in moist cheesecloth, wrapped in aluminum foil, and subjected to –12, –18, –24, or –30C. Cooling was initiated from the storage temperature (2C) at 2 ± 0.5C/h. Treated crowns were allowed to thaw for 24 h at 2 ± 2C. Control and treated crowns then were planted in 3.5-liter (#1) plastic containers using a medium of 2 Canadian sphagnum moss: 2 perlite: 1 field soil (by volume), and transferred to a 21 ± 3C glasshouse for forcing under natural photoperiod. Regrowth data indicate `Ruby Mound' was the least cold hardy selection as most failed to resume growth after exposure to –12C. `Emily', `Megan', and `Triumph' survived exposure to –12C; however, vegetative regrowth was diminished compared to controls. `Baby Tears' and `Debonair' demonstrated no diminution of ornamental utility after exposure to –12C.

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Jeffery K. Iles

Rewholesalers, garden centers, and other sellers of deciduous shrubs routinely receive bare-root stock in late winter or early spring for potting; however, bare-root plants are sometimes slow to establish in containers. Potted liners with well-developed root systems show potential for shortening the production cycle and permitting the development of higher-quality plants earlier in the growing season. To study the effect of nursery stock type and size on subsequent growth, two bare-root sizes and one potted liner size of `Cardinal' red osier dogwood (Cornus sericea L.), `Goldflame' spirea (Spiraea xbumalda Burv.), and `James MacFarlane' lilac (Syringa xprestoniae McKelv.) were grown in polyethylene containers of different sizes. Bare-root plants (15 and 30 cm in height) were grown in 2.7- and 6.1-L, and 6.1- and 10.3-L containers, respectively. Potted liners (0.4-L container size) were grown in 6.1- and 10.3-L containers. Plant performance was evaluated 10 and 20 weeks after potting. In general, plant quality ratings increased with container volume for all species. For `Goldflame' spirea and `James MacFarlane' lilac, best plant quality ratings occurred with 30-cm plants grown in 10.3-L containers. But for `Cardinal' redosier dogwood, plant quality ratings were highest and not significantly different for 30-cm bare-root plants and potted liners grown in 10.3-L containers.

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Jeffery K. Iles

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Jeffery K. Iles and Nancy Howard Agnew

Dormant, intact crowns were used to determine the cold hardiness of the herbaceous perennial Heuchera sanguinea `Chatterbox'. Crowns were placed in moist cheesecloth, wrapped in aluminum foil, and subjected to -4,-6,-8,-10,-12,-14, -16, or -18C in a programmable freezer. Regrowth quality ratings and dry-mass measurement decreased linearly with temperature. No regrowth was evident from any crown exposed to -12C or lower temperatures. Freezing dormant plant crowns proved an efficient and reliable technique for estimating cold hardiness of Heuchera `Chatterbox'.

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Jeffery K. Iles and Nancy H. Agnew

Nine herbaceous perennial species were evaluated for use as flowering pot plants for late winter and early spring sales. Plugs of Achillea `King Edward', Arabis sturii, Armeria `Alba', Bergenia `New Hybrid', Chrysogonum virginianum, Dianthus `War Bonnet', Phlox `Chattahoochee', Platycodon `Sentimental Blue', and Veronica `Sunny Border Blue' were established in 14-cm (0.8-liter) round plastic containers, grown for one season, and covered with a thermoblanket for winter. Five plants of each species were transferred to a 21 ± 3C glasshouse for forcing under natural daylength at six 10-day intervals beginning 1 Dec. 1993. By this date plants had experienced approximately four weeks of temperatures below 5C. Ambis, Chrysogonum, and Phlox, species that naturally flower in spring, were the most floriferous. Days to first flower for Arabis decreased from 30 to 26 while flower number increased 44% by the 20 Dec. forcing date. For Phlox, days to first flower decreased from 36 to 31 by 20 Dec., but flower numbers were similar regardless of forcing date. Chrysogonum averaged eight flowers throughout the study, but days to first flower increased from 25 (1 Dec.) to 31 in all following forcing dates.

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Jeffery K. Iles and Nancy H. Agnew

Nine herbaceous perennial species were evaluated for use as flowering potted plants for late winter and early spring sales. Plugs of `King Edward' Achillea × Lewisii Ingw. (yarrow), Arabis sturii Mottet. (rockcress), `Alba' Armeria maritima (Mill.) Willd. (common thrift), `New Hybrid' Bergenia cordifolia (Haw.) Sternb. (bergenia), Chrysogonum virgianum L. (goldenstar), `War Bonnet' Dianthus × Allwoodii Hort. Allw. (Allwood pinks), Phlox × chattahoochee L. (Chattahoochee phlox), `Sentimental Blue' Platycodon grandiflorus (Jacq.) A. DC. (balloonflower), and Veronica L. × `Sunny Border Blue' (veronica) were established in 14-cm (0.8-liter) round plastic containers, grown for one season and covered with a thermoblanket for winter. Five plants of each species were transferred to a 21 ± 3C glasshouse for forcing under natural daylengths at six 10-day intervals beginning 1 Dec. 1993. Arabis sturii, Phlox × chattahoochee, Platycodon grandiflorus `Sentimental Blue', and Veronica × `Sunny Border Blue' flowered out of season without supplemental lighting. `Alba' Armeria maritima and Chrysogonum virginianum also flowered; however, their floral displays were less effective. `New Hybrid' Bergenia cordifolia did not flower and `King Edward' Achillea × Lewisii and `War Bonnet' Dianthus × Allwoodii only flowered sporadically, therefore, these perennials are not recommended for forcing out of season using our vernalization method.

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Jeffery K. Iles and Nancy H. Agnew

The capacity of plant materials to resume normal growth after exposure to low temperature is the ultimate criterion of cold hardiness. We therefore determined the low-temperature tolerance of five commercially important herbaceous perennial species. Container-grown blanket flower (Gaillardia ×grandiflora Van Houtte. `Goblin'), false dragonhead [Physoste- gia virginiana (L.) Benth. `Summer Snow'], perennial salvia (Salvia ×superba Stapf. `Stratford Blue'), painted daisy (Tanacetum coccineum Willd. `Robinson's Mix'), and creeping veronica (Veronica repens Loisel.) were subjected to 0, -2, 4, -6, -8, -10, -12, -14, -16, and -18C in a programmable freezer. The percentage of survival of most species was adequate when exposed to -10C. Producers of container-grown perennials are advised to provide winter protection measures that prohibit root medium temperatures from falling below -10C.