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  • Author or Editor: Jeffrey H. Gillman x
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Hydrogels are crystals that can suck up 600 or more times their weight in water. These gels are sold as additives for soils and container media for the purpose of reducing the frequency of watering. Five different hydrogels and a control were tested on geranium and 3 different hydrogels and a control were tested on ninebark to see how long plants could be kept healthy without watering. Growth was roughly similar among the control and the different hydrogels tested with the exception of Hydrosorb™, which stunted the growth of the ninebark. After plants reached a size that was considered saleable watering was stopped and the plants were allowed to dry out. None of the hydrogels kept the plants supplied with water for any longer than the controls. Hydrosorb™ did appear to keep ninebarks at a healthy water potential for longer than the other hydrogels and the control, however, this is almost certainly because of the smaller size of the plants.

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Phosphorus contamination of surface water is a growing problem associated with container production of nursery plants. Iron and iron compounds have the ability to adsorb phosphorus and render it immobile. Incorporating iron compounds into media at the base of nursery containers serves to filter out phosphorus from fertilizers while still allowing the plant to collect enough phosphorus to grow. Two experiments were devised. The first experiment examined how much phosphorus various iron compounds would adsorb. Metallic iron adsorbed the most phosphorus, followed by HCl reacted magnetite (a form of iron ore), Fe2O3, Fe3O4 and magnetite. In the second experiment, PVC tubes (4 cm inner diam.) were filled to a level of 5 cm with a phosphorus adsorbing layer containing growing media that was 25% or 50% by weight iron compounds. Compounds included metallic iron, HCl reacted magnetite and magnetite. Plain media was used as a control. A layer of 15 cm of media and slow-release fertilizer was applied above the adsorptive layer. One hundred milliliters of distilled water was applied to PVC tubes daily to simulate irrigation. Metallic iron reduced phosphorus leachate to almost 0 for over 2 weeks. HCl reacted magnetite was also effective in reducing phosphorus leachate. Magnetite only affected phosphorus leachate slightly.

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Plants that have been grown in containers for a long period of time frequently develop roots that grow in circles, following the contour of the container in which they have been planted. This condition is commonly referred to as “pot-bound.” It is considered common knowledge that if a pot-bound plant is transplanted without any treatment, its roots will continue to follow the contour of the now-removed container. There are, however, a number of transplanting techniques that are intended to reorient the roots in a direction that will be conducive to helping roots to grow out of this potentially harmful situation. These techniques include: butterflying, or slicing the rootball into two halves before planting; scoring, or making inch-deep slices around the rootball at 90° increments and an X-shaped slice across the bottom; or teasing, where roots are manually pulled out of the shape of the container in a direction perpendicular to the stem. Severely pot bound Salixalba and Tiliacordata were treated with one of the three treatments previously listed or as a control and were transplanted into an experimental field and grown for two full seasons. After two seasons, the trees were harvested and the number and size of roots escaping from the pot-bound region were recorded. None of the treatments allowed roots of any size to escape the pot-bound mass more effectively than the control.

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Roses in nursery and landscape settings are frequently damaged by black spot, whose causal agent is the fungus Diplocarpon rosae F.A. Wolf. Potassium silicate was assessed as a media-applied treatment for decreasing the severity and incidence of black spot infection. Roses were treated with 0, 50, 100, or 150 mg·L-1 silicon as potassium silicate incorporated into irrigation water on either a weekly or daily schedule. Five weeks after treatments were initiated, plants were inoculated with D. rosae. Roses began to show visual symptoms of infection §4 days later. Roses that had 150 mg·L-1 silicon applied on a daily schedule had significantly more silicon present in their leaves than other treatments as measured by scanning electron microscopy and energy-dispersive x-ray analysis. In addition, roses that had 100 and 150 mg·L-1 silicon applied on a daily schedule had fewer black spot lesions per leaf and fewer infected leaves than any of the other treatments by the end of the experiment 7 weeks later. Although roses treated with higher levels of silicon on a daily basis fared better than roses in the other treatments, all of the roses were heavily infected with D. rosae by the end of the study. The results reported here indicate that using potassium silicate in irrigation water may be a useful component of a disease management system.

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Hybrids of Corylus avellana, C. americana and C. cornuta, are being developed as a potential crop for the Upper Midwest of the United States, but little is known about fertilizer nitrogen (N) management. We hypothesized that N application when the bushes were most fully leafed out would result in highest N uptake efficiency (NUE). We used 15N-labeled ammonium nitrate to measure NUE from soil applications in mid-April, late April, late May, early August, and mid-September. Nitrogen applied in either mid- or late April never comprised more than 5% of the total N in shoots or leaves, suggesting that N used for early leaf expansion came primarily from stored reserves. Applications made after April demonstrated that N was quickly translocated to rapidly growing plant parts: May applications comprised 9% of the N in leaves collected in July; August applications comprised 12% of the N in nut kernels collected in September; and September applications comprised 9% of N in catkins collected in October. Nitrogen applied in August and September appeared in new shoots the following April at higher levels than it did aboveground the previous October, showing that N applied late in the season may be stored belowground over the winter. NUE was highest for August and September applications at one site and August and mid-April applications at the other, implying that summer is generally the best time to apply N for most efficient uptake. However, overall NUE was low, only 5% for August applications, suggesting a need to develop other methods of improving NUE in hybrid hazelnuts.

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In Fall 1999, the University of Minnesota implemented a writing-intensive (WI) requirement for undergraduates. As part of the requirement, students must take one upper-division WI course in their major. As of Spring 2002, the environmental horticulture major through the Department of Horticultural Science had only one WI course in its entire curriculum. Teaching faculty were interviewed and syllabi were reviewed to gather information on what types of writing are currently being assigned and to discuss where more WI courses should be placed in the environmental horticulture curriculum in the future. These surveys and interviews revealed that the majority of classes require formal writing and that the majority of the faculty review or are willing to review a draft of an assignment, two key components of the WI requirement. Informal writing assignments are less common, indicating a deficient area of the curriculum. With slight modifications, many classes in the environmental horticulture curriculum can meet the requirements to become designated as WI. Faculty agreed that WI courses should be placed in upper-level, smaller classes that place less emphasis on production techniques or plant identification.

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Buddleia taxa were assessed for two-spotted spider mite (Tetranychus urticae Koch) resistance using a leaf disk bioassay, a novel shell vial bioassay and a field trial. Leaf pubescence and chemistry were examined for their role in two-spotted spider mite resistance. Results from bioassays and field sampling identified highly resistant taxa including B. fallowiana Balif. `Alba' and B. davidii × B. fallowiana Franch. `Cornwall Blue' as well as susceptible taxa including B. davidii Franch. `African Queen' and B. lindleyana Fort. ex Lindl. `Gloster'. The shell vial bioassay was an accurate predictor of field resistance to spider mite. Leaf pubescence was quantified by calculating the collective length of trichome branches per square millimeter of leaf surface area [effective branch length (EBL)]. EBL values ranged from 39 to 162 mm·mm-2 of leaf surface area among Buddleia taxa. Resistance was positively correlated with increased pubescence. Removal of pubescence by peeling resulted in increased oviposition of two-spotted spider mites. Exposing female two-spotted spider mites to a methylene chloride extract of B. davidii × B. fallowiana `Cornwall Blue' using a modified shell vial bioassay resulted in reduced oviposition and a methylene chloride extract of B. davidii `African Queen' resulted in no difference in oviposition when compared with a control. While pubescence is the best indicator of resistance to the two-spotted spider mite in Buddleia taxa, it is possible that defensive compounds are involved.

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Two experiments were conducted to determine the effect of drought stress on the susceptibility of Buddleia davidii Franch. `Pink Delight' to the two-spotted spider mite (Tetranychus urticae Koch). In the first experiment, drought stress was imposed by withholding water until predawn xylem pressure potential fell below -1 MPa. Shoot growth was 75% less in drought-stressed than in nonstressed plants. Mite population densities were not affected, but noninfested leaf area was 14% higher, and degree of mite damage was lower, in nonstressed plants. Evidently, the greater amount of new growth in nonstressed plants leads to lower spider mite densities by diluting populations. In a second experiment, nonstressed B. davidii `Pink Delight' plants were watered every 1 to 2 days and drought-stressed plants were watered every 3 days. Spider mite populations were monitored by sampling newly expanded and mature foliage. Mite populations on mature foliage were not affected by stress, but stressed plants grew less and had larger spider mite populations on their newly expanded foliage than did nonstressed plants.

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