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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.

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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

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

Electrolyte leakage and regrowth were measured from September through January to determine cold hardiness of Sedum spectabile × telephium L. `Autumn Joy' and Sedum spectabile Boreau. `Brilliant' plants grown outdoors in central Iowa. Crowns were subjected to 0, –3, –6, –9, –12, –15, –18, –21, –24, or –27C. Regrowth tests were performed on whole crowns and electrolyte leakage was determined on excised tuberous root and crown tissue. Both cultivars were killed at –3C in September, but they acclimated gradually through January. Maximum hardiness was achieved in January, with killing temperatures of –27C for `Autumn Joy' and –21C for `Brilliant'. Regrowth quality ratings were significantly correlated with crown and tuberous root electrolyte leakage measurements, although the relationship was stronger for `Autumn Joy'.

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Ai-Yu Chang, Mark L. Gleason, Nancy H. Agnew, Dennis G. Olson and Richard T. Gladon

Irradiated cut Rosa × hybrida `Royalty' flowers were used to determine the efficacy of electron-beam irradiation for extending flower postharvest life by reducing native and inoculated populations of Botrytis cinerea. In preliminary experiments, roses received irradiation dosages of 0.00, 0.50,1.00, 2.00, and 4.00 kilogray (kGy), along with an untreated control, to establish killing dosages. Irradiation dosages of 1.00 kGy or greater irreversibly damaged rose petal tissue. In subsequent experiments, roses irradiated at dosages of 0.00, 0.25, 0.50, 0.75, and 1.00 kGy, and an untreated control, were used for evaluating postharvest events. We have found that irradiation dosages of 0.25 and 0.50 kGy slowed the rate of flower bud opening slightly and did not decrease postharvest quality or longevity. Inoculated and uninoculated roses irradiated at 0.00, 0.25, 0.50, and 0.75 kGy were used to determine if electron-beam irradiation could reduce Botrytis infection and proliferation during postharvest storage, and these results also will be presented.

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Ai-Yu Chang, Richard J. Gladon, Mark L. Gleason, Sharon K. Parker, Nancy H. Agnew and Dennis G. Olson

Cut Rosa ×hybrida L. `Royalty' flowers were used to determine the efficacy of electron-beam irradiation for increasing postharvest quality and decreasing petal infection by Botrytis cinerea Pers. In an experiment for determining the injury threshold, roses received electron-beam irradiation of 0, 0.5, 1, 2, and 4 kGy. Irradiation dosages ≥1 kGy caused necrosis on petal tissue and decreased postharvest life at 20 °C. In a second experiment to evaluate postharvest quality, roses were irradiated at 0, 0.25, 0.5, 0.75, and 1 kGy. Dosages of 0.25 and 0.5 kGy slowed the rate of flower bud opening for 2 days but did not decrease postharvest quality when compared with nonirradiated roses. Roses that received irradiation dosages of 0.75 and 1 kGy showed unacceptable quality. In a third experiment, roses that had or had not been inoculated with B. cinerea were irradiated at 0, 0.25, 0.5, and 0.75 kGy. Irradiation did not control B. cinerea populations, and rose quality decreased as dosage increased. In a fourth experiment to determine the effect of irradiation on B. cinerea, conidia on water-agar plates exposed to dosages ≤1, 2, and 4 kGy germinated at rates of ≈90%, 33%, and 2%, respectively, within 24 h.

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Richard.J. Gladon, Cheryll A. Reitmeier, Mark L. Gleason, Gail R. Nonnecke, Nancy H. Agnew and Dennis G. Olson