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J. Ryan Stewart and Roger Kjelgren

Infrared sensors were used to quantify canopy temperature and thus detect differences in incipient water stress between a cool-season grass [Kentucky bluegrass (KBG) (Poa pratensis)] and a warm-season grass [buffalograss (BG) (Buchloe dactyloides)]. The infrared sensors, connected to a datalogger, measured average hourly leaf–air temperatures (TL–TA) 1 m above eight replicate plots of Kentucky bluegrass and eight replicate plots of buffalograss. Air temperature and relative humidity from a nearby weather station were used to calculate the average hourly vapor pressure deficit (VPD). In late July, we ceased irrigating and measured TL–TA and soil water content while allowing the turf to dry down for 5 weeks. Soil water content was measured with a neutron probe. Both species exhibited a significant relationship between TL–TA and VPD. As the VPD increased, TL–TA decreased in both species (KBG r 2 = 0.73, BG r 2 = 0.71) on the 2nd day after an irrigation during well-watered conditions. An artifact was created on the first day after an irrigation as a result of excessive surface evaporation. KBG and BG were similar under well-watered conditions. KBG had a higher TL–TA after 4 to 5 days without irrigation. By contrast, BG did not have a higher TL–TA until 25 to 30 days without irrigation. Part of BG's drought avoidance was extraction of soil water down to 0.9 m vs. 0.45 m for KBG.

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J. Ryan Stewart and Irene McGary

Although there is increasing interest in propagating prairie plants native to the midwestern United States for managed and natural landscapes, several species, including new jersey tea (Ceanothus americanus), are difficult to germinate from seeds. New jersey tea, which is an attractive, compact woody shrub, is found in high-quality prairie remnants throughout the tallgrass prairie region. Developing a protocol to increase the uniformity of seed germination in this species would allow for more widespread horticultural cultivation of this stress-resistant, nitrogen-fixing species. We hypothesized that the germination response of seeds of new jersey tea would be enhanced by replicating conditions that mimic their natural environment, which included treatments under controlled conditions exposing seeds to chilling temperatures, sulfuric acid, and boiling water. Two minutes of exposure to boiling water followed by 60 days of cold-moist stratification at 4 °C resulted in the highest germination percentage (48%) and mean daily germination (2.18 seeds/day). Scarification with 98% sulfuric acid for 15 min followed by 60 days of cold-moist stratification resulted in significant, but lower levels of germination percentage than seeds exposed to boiling water and cold-moist stratification. Cold-moist stratification in darkness and in an 18-hour photoperiod at 4 °C did not stimulate germination to a level suitable for production purposes. However, tetrazolium tests indicated that 79% of the seeds were viable. We conclude that cold-moist stratification should be used with boiling-water or acid-scarification to uniformly produce germinated seedlings of new jersey tea. However, nonresponsive seeds should not be discarded because they may germinate in later years if kept under appropriate conditions.

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J. Ryan Stewart and William R. Graves

Rhamnus caroliniana Walt. (carolina buckthorn or indian cherry) is an attractive small tree or shrub found in diverse habitats in the United States. Because the species occurs in both mesic and xeric soils, we questioned whether selections of carolina buckthorn could be marketed as new nursery crops resistant to both drought and flooding. Our first objective was to characterize how soil water affects growth and gas exchange of carolina buckthorn. We studied potted plants subjected to soil moistures that ranged from complete submersion of the root zone to severe drought (7% soil water by volume). The maximal photosynthetic rate occurred at 27% soil water content, and complete submersion killed plants. Our second objective was to compare responses of carolina buckthorn to those of the invasive common buckthorn (Rhamnus cathartica L.) when potted plants were treated with partial flooding of root zones and drought. Carolina buckthorn resisted deleterious effects of partial flooding. In contrast, leaves of common buckthorn became epinastic, and rates of photosynthesis were low (2.14 μmol CO2/m2/s) after 17 days of treatment. Mean photosynthesis of common buckthorn increased to 5.52 μmol CO2/m2/s, a rate similar to that of carolina buckthorn, after 55 days of treatment. Drought reduced net photosynthesis by 52% and 68%, respectively, for carolina buckthorn and common buckthorn relative to rates of plants in the control treatment. We conclude that carolina buckthorn is capable of maintaining carbon fixation and growth over a wide range of soil water contents, and unlike common buckthorn, is not dependent upon morphological, anatomical, or physiological adjustments to optimize growth and net photosynthesis in extremely wet soil. Use of carolina buckthorn as an ornamental is warranted if invasiveness and other potential problems with the species are not identified.

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J. Ryan Stewart and William R. Graves

Little is known about the reproductive biology of carolina buckthorn [Rhamnus caroliniana Walt. or Frangula caroliniana (Walt.) Gray], an attractive North American shrub or small tree that might merit increased use in managed landscapes. The fecundity and high germinability of seeds of the Eurasian common buckthorn (Rhamnus cathartica L.), however, have been characterized as factors contributing to its invasiveness. We compared seed germination of these species to ascertain how easily carolina buckthorn could be grown from seed in nurseries and to acquire data for predicting whether carolina buckthorn might be invasive if introduced into managed landscapes. Fruits of carolina buckthorn were collected from indigenous plants in central Missouri, southern Oklahoma, and southern Texas. Fruits of common buckthorn were collected from shrubs naturalized in central Iowa. Seeds of both species were stratified for up to 112 days in darkness at 4 °C; germination at 24 °C in the dark was then evaluated for 56 days. Quadratic functions best described how time of stratification influenced germination value and germination percentage of common buckthorn, whereas these measures of carolina buckthorn were best represented by exponential (value) or linear (percentage) functions. Stratification for 112 days maximized germination value and percentage for carolina buckthorn within the 56-day germination period, but shorter stratifications were sufficient to optimize germination of common buckthorn. While the overall mean germination of carolina buckthorn was 40%, results varied by provenance and ranged from 25% (Missouri) to 56% (Oklahoma). Mean germination of common buckthorn over times of stratification was 71%, and the overall mean daily germination of common buckthorn, 1.3, was 86% greater than that of carolina buckthorn, 0.7. We conclude that seeds of carolina buckthorn are more resistant to germination than seeds of common buckthorn. Our results suggest that plant propagators should cold-stratify seeds of carolina buckthorn for up to 112 days, and suggest that carolina buckthorn has a lower potential to be invasive than does common buckthorn.

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J. Ryan Stewart* and William R. Graves

Some buckthorn species from other continents have proven invasive in North American landscapes. Carolina buckthorn (Rhamnus caroliniana Walt.) is an attractive, native species that would merit increased use in horticultural landscapes if concerns about its potential invasiveness are allayed. Invasiveness often is associated with efficient use of water and other resources. We tested for differences between Carolina buckthorn and common buckthorn (Rhamnus cathartica L.) in photosynthesis, aboveground dry matter accumulation, and water-use efficiency. Seedlings were grown in columns of field soil within insulated pots outdoors for 98 days. Net photosynthesis of Carolina buckthorn was 17% to 39% greater than that of common buckthorn through day 22. This difference between species was reversed through the end of the treatment period with a concomitant increase in leaf temperature of Carolina buckthorn. Final dry weight of aboveground tissues was similar for the two species, but a greater proportion of dry matter was partitioned to stems for common buckthorn compared to Carolina buckthorn. Although common buckthorn initially had higher water-use efficiency (110 mg·g-1 per day) than did Carolina buckthorn (60 mg·g-1 per day), the water-use efficiency of both species decreased to similar values for the remainder of the treatment period. We conclude that young plants of common buckthorn do not use water more efficiently than do young Carolina buckthorn under field conditions in central Iowa. Considering the possible species differences in the relationship between temperature and photosynthesis, comparative water-use efficiency should be tested further in other environments where Carolina buckthorn might be used for landscaping.

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J. Ryan Stewart* and William R. Graves

Carolina buckthorn (Rhamnus caroliniana Walt.) is ornamental and could be promoted as a stress-resistant shrub for horticultural landscapes. Its status as a relative of invasive species, including common buckthorn (Rhamnus cathartica L.), raises concerns regarding the environmental consequences of planting Carolina buckthorn outside of its natural habitat. To assess the ease of propagating Carolina buckthorn from seed, and to gather data relevant to assessments of invasiveness, we compared seed-germination characteristics between the two species. Seeds of Carolina buckthorn were collected from native populations in Missouri, Oklahoma, and Texas. Seeds of common buckthorn were collected from populations in Iowa. We stratified seeds of both species for up to 112 days at 4 °C. Germination at 20 °C then was evaluated for 56 days. Over stratification durations, 40% and 71% of seeds of Carolina buckthorn and common buckthorn germinated, respectively. Stratification for 112 days optimized germination value for Carolina buckthorn, but stratification for 42, 56, 84, and 112 days evoked similar germination percentages. Seeds of Carolina buckthorn from Oklahoma germinated at a higher percentage (56%) than did seeds from Missouri (25%). Neither germination value nor germination percentage of common buckthorn was influenced by stratification. We conclude that seeds of Carolina buckthorn are more recalcitrant than are seeds of common buckthorn. This suggests that Carolina buckthorn, particularly those from Missouri with low reproductive success, may be less invasive than their Eurasian kin. Horticulturists can optimize germination percentage of Carolina buckthorn by cold-stratifying seeds for as little as 42 days, but 112 days optimizes germination value.

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Steven J. Bergsten, Andrew K. Koeser and J. Ryan Stewart

Long used as a source of food, beverages, and fiber, Agave exhibits potential to be cultivated as a crop to produce alternative sweeteners, bioenergy, and a variety of other end uses. However, little is known regarding the productivity levels of Agave when grown in saline soils in semiarid regions. Hydroponic experiments were carried out to evaluate the effects of salinity on biomass accumulation and nutrient levels of young plants of Agave parryi, Agave utahensis ssp. kaibabensis, Agave utahensis ssp. utahensis, and Agave weberi. Salinity treatments (0.6, 3.0, 6.0, and 9.0 dS·m−1) were imposed in each experiment. Both subspecies of A. utahensis were sensitive to salt treatments. In the higher salinity treatments, A. utahensis ssp. utahensis exhibited high mortality; both subspecies had lower plant dry weights. Agave parryi was more tolerant, but experienced a decrease in plant dry weight in the 9.0 dS·m−1 treatment. The biomass of A. weberi was unaffected by any level of salinity. Calcium, Mg, S, and Mn levels decreased in both A. parryi and A. weberi at higher salinity levels. Potassium and P levels in A. parryi decreased in the higher salt treatments. Decreases in nutrients were not severe enough to cause any apparent nutrient deficiencies in A. parryi and A. weberi. Agave parryi and A. weberi tolerated salinity at higher levels than expected, and may show promise for cultivation in saline soils.

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J. Ryan Stewart, William R. Graves and Reid D. Landes

Can Carolina buckthorn (Rhamnuscaroliniana) persist north of its native habitat without becoming invasive? Its distribution (USDA zones 5b to 9b) suggests that genotypes vary in cold hardiness, and invasiveness of other Rhamnus sp. has been linked to unusually early budbreak each spring. Therefore, we investigated depth of cold hardiness and vernal budbreak of Carolina buckthorns from multiple provenances and made comparisons to the invasive common buckthorn (Rhamnus cathartica). Budbreak was recorded in Ames, Iowa, from 9 Apr. to 10 May 2002. Buds of common buckthorn broke earlier than those of Carolina buckthorn, and mulching plants of Carolina buckthorn hastened budbreak. Stem samples were collected in October, January, and April from a plot in Ames, Iowa (USDA zone 5a), of Carolina buckthorns from three provenances (Missouri, Ohio, and Texas) and of naturalized common buckthorns. A similar schedule was followed during the next winter, when two plot locations [Ames, Iowa, and New Franklin, Mo. (USDA zone 5b)], were compared, but Carolina buckthorns from only Missouri and Texas were sampled. Carolina buckthorn and common buckthorn survived midwinter temperatures as low as –21 °C and –24 °C, respectively. Provenance differences were minimal; Carolina buckthorns from Missouri were more hardy than those from Ohio and Texas only in April of the first winter. We conclude that its cold hardiness will permit use of Carolina buckthorn beyond where it is distributed in the southeastern United States. Delayed budbreak of Carolina buckthorn relative to that of common buckthorn may underscore the potential for Carolina buckthorn in regions with harsh winters and may lessen its potential to be as invasive as common buckthorn.

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Andrew Koeser, Sarah T. Lovell, Michael Evans and J. Ryan Stewart

In recent years, biocontainers have been marketed as sustainable alternatives to petroleum-based containers in the green industry. However, biocontainers constructed with plant materials that are highly porous in nature (e.g., peat, wood fiber, straw) tend to require more frequent irrigation than conventional plastic products. As irrigation water sources become less abundant and more expensive, growers must consider water consumption in any assessment of their economic and environmental viability. This project evaluated plant growth and total water consumption for nine different biocontainers (seven organic alternatives, and two recently developed bioplastic alternatives) and a plastic control used to produce a short-term greenhouse crop, ‘Yellow Madness’ petunia (Petunia ×hybrida). Dry shoot weight and total water consumption differed by container type, with some of the more porous containers (wood fiber, manure, and straw) requiring more water and producing smaller plants by the end of the trial period. Intuitively, the more impervious plastic, bioplastic, and solid rice hull containers required the least irrigation to maintain soil moisture levels, though shoot dry weights varied among this group. Shoot dry weight was highest with the bioplastic sleeve and slotted rice hull containers. However, the latter of these two containers required a greater volume of water to stay above the drying threshold. Findings from this research suggest the new bioplastic sleeve may be a promising alternative to conventional plastic containers given the current production process.

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J. Ryan Stewart, William R. Graves and Reid D. Landes

Carolina buckthorn [Rhamnus caroliniana Walt. or Frangula caroliniana (Walt.) Gray] is an attractive and water-stress-resistant shrub or small tree distributed extensively in the southeastern United States that merits use in managed landscapes. Due to substantial climatic differences within its distribution (30-year normal midwinter minima range from 13 to -8 °C), selection among provenances based on differences in cold hardiness is warranted. Before selections are marketed, the potential of carolina buckthorn to be invasive also merits investigation. Ecological problems resulting from the introduction of Rhamnus L. species in the United States, most notably the dominance of R. cathartica L. (common buckthorn) over neighboring taxa, are due in part to early budbreak. Consequently, we investigated depth of cold hardiness and vernal budbreak of carolina buckthorn and common buckthorn. Stem samples of carolina buckthorn and common buckthorn collected in midwinter survived temperatures as low as -21 and -24 °C, respectively. Although the cold hardiness of carolina buckthorns from Missouri was greater than that of carolina buckthorns from Ohio and Texas on 2 Apr. 2003, there were no differences in cold hardiness of stems from Missouri and Texas on all three assessment dates in the second experiment. All plants survived at both field locations except for the carolina buckthorns from southern Texas planted in Iowa, which showed 0% and 17% survival in 2003 and 2004, respectively. Budbreak of both species with and without mulch in Ames, Iowa, was recorded from 9 Apr. to 10 May 2002. Mean budbreak of common buckthorn was 5.7 days earlier than budbreak of carolina buckthorn, and buds of mulched carolina buckthorns broke 4.2 days earlier than did buds of unmulched carolina buckthorns. We conclude that the cold hardiness of carolina buckthorn is sufficient to permit the species to be planted outside of its natural distribution. Populations of carolina buckthorn in Ohio and Missouri should be the focus of efforts to select genotypes for use in regions with harsh winters. Phenology of its budbreak suggests carolina buckthorn will not be as invasive as common buckthorn, but evaluation of additional determinants of invasiveness is warranted.