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Richard P. Regan

Applying only the amount of water needed by a plant, when it needs it, is a simple concept that can conserve water and reduce runoff. Simple, that is, when managing a single crop that covers an extensive area under several irrigation zones. Container production nurseries grow a large number of plants and each irrigation zone usually has a diverse grouping of taxa in various stages of development. In 1989, a nursery crop project at Oregon State University began to investigate irrigation scheduling for container-grown woody landscape plants. Crop coefficients (kc), used to adjust irrigation to specific production practices and crop characteristics, vary greatly for woody landscape plants. Woody plant kc values range from <1.0 to >5.0 during the production cycle. Plant taxa, growth stage, spacing, and pruning significantly influence kc of container-grown plants. Ilex crenata `Green Island' showed a reduction in water use (40%) immediately after pruning, but had similar kc values 60 days later. Grouping plants with similar kc values under the same irrigation zone is a very difficult task for a production nursery. It might be more practical to schedule irrigation for daily evapotranspiration, avoid placing new plantings next to mature crops, and only separate-out plants with very high or very low crop water requirements.

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Carolyn F. Scagel, Richard P. Regan and Guihong Bi

A study was conducted to determine whether the nitrogen (N) status of nursery-grown green ash (Fraxinus pennsylvanica ‘Summit’) trees in the autumn is related to bud necrosis during the following spring. In 2005, different rates of N from urea formaldehyde (UF) or a controlled-release fertilizer (CRF) containing ammonium nitrate were applied during the growing season to green ash trees and leaves were sprayed or not with urea in the autumn. Biomass and N content was determined in Autumn 2005 and Spring 2006, and stem biomass and bud necrosis were evaluated for necrosis in Spring 2006. Trees with low N content in Autumn 2005 grew less in Spring 2006 but bud necrosis was more prevalent on trees grown at the highest N rate. Compared with trees grown with a similar amount of N from UF, growing trees with CRF altered N allocation in 2005 and the relationship between carbon (C) and N dynamics (import, export, and metabolism) in stems in 2006. Additionally, trees grown with CRF had less total shoot biomass in Spring 2006 and more bud failure than trees grown with a similar N rate from UF. Significant relationships between bud failure and N status and C/N ratios in different tissues suggest that a combination of tree N status and the balance between N and C in certain tissues plays a role in the occurrence of bud failure of green ash trees in the spring.

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Carolyn F. Scagel, Richard P. Regan, Rita Hummel and Guihong Bi

A study was conducted to determine whether nitrogen (N) application rate and fertilizer form are related to cold tolerance of buds and stems using container-grown ‘Summit’ green ash (Fraxinus pennsylvanica) trees. Trees were grown with different rates of N from either urea formaldehyde (UF) or a controlled-release fertilizer (CRF) containing ammonium nitrate during the 2006 growing season; and growth, N and carbon (C) composition, and cold tolerance were evaluated in Oct. 2006, Dec. 2006, and Feb. 2007 by assessing the lowest survival temperature (LST) of stem and bud tissues on current season (2006) stems. Both fertilizer type and rate influenced the bud and stem LSTs. The influence of fertilizer rate was most evident on midwinter (December) stem LSTs and the influence of fertilizer type was observed in bud and stem LSTs during the deacclimation period in February. Higher LSTs were associated with higher N concentrations and lower C/N ratios; however, stems and buds of trees fertilized with UF were more cold-tolerant (had lower LSTs) than stems and buds on trees fertilized with CRF. Fertilizer type resulted in several differences in N and C translocation and metabolism during the fall and winter. Our results indicate trees with a similar N status are able to withstand different levels of cold depending on the rate of N and the type or form of fertilizer used during production. This may have to do with differences in how trees metabolize the different fertilizer forms, where and when the N is stored, and how it is remobilized in the spring, especially in relation to C metabolism.

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

The influence of nitrogen (N) fertilizer application on plant allocation, uptake, and demand for other essential nutrients was evaluated from May 2005 to Feb. 2006 in evergreen Rhododendron ‘P.J.M. Compact’ (PJM) and ‘English Roseum’ (ER) and deciduous Rhododendron ‘Gibraltar’ (AZ) grown in containers filled with soilless substrate. Net nutrient uptake and losses were calculated using piecewise regression and uptake efficiency, root absorption capacity, aboveground demand, nutrient use efficiency, and uptake ratios between N and other nutrients (N ratios) were calculated using net uptake between harvest dates. Nitrogen application increased uptake rate of all nutrients, enhanced late-season uptake of many nutrients, and increased the rate of nutrient loss during the winter. Nutrient uptake often occurred as late as November in plants grown with N but was usually undetectable after September in plants grown without additional N fertilizer. Nutrient losses during the winter were not always associated with biomass loss and were related to differences in preferential nutrient allocation to different structures and the plant's ability to export nutrients before biomass loss. Plants with a greater potential for rapid growth were more capable of later-season nutrient uptake than plants with slower growth rates. Nitrogen availability altered N ratios indicating that when adding N to container-grown Rhododendron, fertilizers with higher ratios of N/phosphorus (PJM, AZ), N/calcium (PJM, ER), N/boron (PJM AZ), N/copper (PJM, ER), and N/iron (PJM, ER) and lower ratios of N/potassium (PJM, ER, AZ), N/sodium (PJM, ER, AZ), N/calcium (AZ), N/boron (ER), N/manganese (AZ), and N/zinc (ER) may be needed to optimize growth and minimize nutrient inputs. Increasing N availability altered uptake efficiency, root absorption capacity, aboveground demand, and nutrient use efficiency for several nutrients, indicating that changes in N management practices need to consider how altering N application rates may influence the plant's ability to take up and use other nutrients. This information can be used to develop fertilizer formulations to minimize excess application of nutrients and to evaluate the potential effects of altering N management practices on use of production resources. Our results indicate that nutrient management strategies for perennial crops such as Rhododendron need to take into consideration not only the nutrient demand for current growth, but also how to optimize nutrient availability for uptake that contributes to future growth potential and end-product quality.

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

The influence of irrigation frequency (same amount of water per day given at different times) on nutrient uptake of container-grown evergreen Rhododendron ‘P.J.M. Compact’ (PJM) and ‘English Roseum’ (ER) and deciduous Rhododendron ‘Gibraltar’ (AZ) grown with different rates of nitrogen (N) fertilizer was evaluated. Increased N application rate increased nutrient uptake and plant dry biomass. Irrigation frequency did not significantly influence total plant dry biomass; however, more frequent irrigation decreased net uptake of several nutrients including phosphorus (P), boron (B), and manganese (Mn) uptake in all cultivars; potassium (K), copper (Cu), and zinc (Zn) uptake in AZ and ER; sulfur (S) uptake in ER and PJM; and iron (Fe) uptake in AZ. Additionally, more frequent irrigation of evergreen cultivars increased calcium (Ca) uptake. Covariate analyses were used to compare nutrient uptake among cultivars and irrigation treatments after accounting for the variability in nutrient uptake attributable to differences in biomass and N uptake. For most nutrients, the influence of irrigation frequency on uptake was partially attributable to differences in biomass and N uptake. After accounting for the variability in nutrient uptake associated with biomass or N uptake, increased irrigation frequency decreased P, S, B, Cu, and Mn uptake only in ER and increased Ca uptake in the two evergreen cultivars. Differences in nutrient uptake among cultivars in response to irrigation treatments were related to water and N availability during production and their combined influence on water stress, nutrient uptake, and biomass partitioning. Estimates of nutrient demand and uptake efficiency using nutrient concentrations and ratios are discussed in relation to nutrient management differences for different cultivars and irrigation treatments.

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Carolyn F. Scagel, Guihong Bi, David R. Bryla, Leslie H. Fuchigami and Richard P. Regan

One deciduous cultivar of Rhododendron L., Gibraltar (AZ), and two evergreen cultivars, P.J.M. Compact (PJM) and English Roseum (ER), were grown in containers for 1 year to determine the effects of irrigation frequency during container production on plant performance the next spring when the plants were transplanted into the landscape. While in the containers, each cultivar was irrigated once or twice daily, using the same amount of water per day, and fertilized with complete nutrient solutions containing 0, 35, 70, or 140 mg·L−1 nitrogen (N). Three months after transplanting into the landscape, nutrient uptake, growth, and flowering were evaluated. In general, the effects of irrigation frequency in containers on performance in the landscape differed between the deciduous cultivar and the evergreen cultivars. In AZ, less frequent irrigation in containers had a pre-conditioning effect that resulted in greater vegetative growth in the landscape but less reproductive growth. In contrast, less frequent irrigation reduced vegetative growth of evergreen cultivars in the landscape and improved flowering. Different growth responses to irrigation frequency between deciduous and evergreen cultivars appeared to be related to differences in timing of nutrient uptake and mobilization. In the deciduous cultivar, less frequent irrigation increased nutrient reserves and improved the ability of the plants to absorb and use nutrients after transplanting, but in the evergreen cultivars, it generally decreased nutrient uptake after transplanting. Less frequent irrigation also altered plant attributes that are important to consumers, including developing a sparser canopy in ER and a fuller canopy in PJM, and producing more but smaller inflorescences in both cultivars. Landscape performance was related to plant nutrition in containers; however, irrigation frequency in containers disrupted relationships between nutrition and performance in all three cultivars. Our results indicate that irrigation frequency during container production of Rhododendron results in a tradeoff between vegetative and reproductive growth the next spring when the plants are in the landscape.

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

The influence of irrigation frequency (same amount of water per day given at different times) and nitrogen (N) fertilizer rate on water stress [stomatal conductance (g S)], N uptake, and growth (biomass) of container-grown evergreen Rhododendron ‘P.J.M. Compact’ and ‘English Roseum’ and deciduous Rhododendron ‘Gibraltar’ was evaluated. Both N deficiency and high N rate increased water stress. Water stress was greatest in plants fertilized with the highest N rate and g S of plants grown with the higher N rates changed more in response to water deficits resulting from irrigation treatments and seasonal climatic changes. Watering plants more frequently decreased water stress of plants fertilized with higher N rates and altering irrigation frequency had little impact on alleviating water stress of N-deficient plants. Increasing irrigation frequency decreased N uptake efficiency (N uptake per gram N applied), increased N use efficiency (growth per gram N uptake) and altered biomass allocation with little influence on total plant biomass. Response of biomass allocation to N rates was similar among cultivars and response of biomass allocation to irrigation frequency varied among cultivars. Altering irrigation frequency changed either the availability of N in the growing substrate or the ability of roots to absorb N. Our results indicate that transitory increases in plant water stress can alter N uptake, N use, and plant form without detectable changes in total plant biomass.

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

The influence of fall sprays with urea on the uptake of nutrients other than nitrogen (N) was assessed using 1-year-old container-grown Rhododendron L. (Rhododendron ‘H-1 P.J.M’) and azalea (Rhododendron ‘Cannon's Double’) grown with different rates of N. Plants were grown with a complete fertilizer containing different N rates from May to Sept. 2004 sprayed or not with urea in the fall of 2004 and grown with a complete fertilizer containing different N rates in the spring of 2005. Urea sprays altered uptake of nutrients other than just N although fertilizer application with other nutrients ceased before plants were sprayed with urea. Across a wide range of plant sizes and N status, urea sprays increased net phosphorus (P), copper (Cu), and manganese (Mn) uptake and decreased net potassium (K) and magnesium (Mg) uptake during the year of urea application. Spraying plants with urea altered nutrient demand and storage in different plant structures during the winter. For azalea, urea sprays increased P demand by roots, Mn demand by 2004 stems, and Cu demand by stems. Urea also decreased storage of K in roots and 2004 stems of azalea and Mg in roots. For rhododendron, urea sprays increased P demand by 2003 stems and 2004 leaves and Mn demand by 2004 leaves. Urea sprays also decreased storage of K and Mg in 2004 leaves of rhododendron. For both cultivars, urea sprays increased mobilization of iron (Fe) from storage and demand for Fe in stems. Spraying Rhododendron with urea in the fall altered uptake and demand for certain nutrients during the following spring. Urea sprays in the fall of 2004 increased uptake and possibly demand for P, K, and sulfur during the spring of 2005 for both cultivars, the uptake of calcium by rhododendron, and the uptake of Mg and Mn by azalea. Our results indicate that when growers spray plants with urea in the fall, spring fertilizer practices may need to be modified to account for increased uptake or demand of certain nutrients.

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami and Richard P. Regan

Growth, nitrogen (N) uptake, and N storage were assessed in transplanted 1-year-old rhododendron liners. Two evergreen cultivars, Rhododendron ‘P. J. Mezitt Compact’ (PJM) and R. ‘English Roseum’ (ER), and one deciduous cultivar, R. ‘Gibraltar’ (AZ), were transplanted into 1-gal. pots and given liquid fertilizer with (+N) or without (–N) N. Increased N availability increased growth after July (ER, PJM) or August (AZ), and resulted in three to five times more total biomass. Biomass continued to increase after stem elongation and leaf production ceased. Nitrogen uptake was correlated with growth of all plant structures on AZ, whereas N uptake was only correlated with stem and leaf growth on evergreen cultivars. The rate of N uptake was highest before July for AZ (1.9 mg·d−1) and in August and September for the evergreen cultivars (≈5 mg·d−1). Thirteen percent to 16% of total N uptake from between May and February occurred after N fertilization ceased at the beginning of September. Plants contained the most N in October (AZ), November (PJM), or December (ER). Biomass loss after November accounted for a loss of 14% to 48% of the maximum total plant N content. Nitrogen demand by roots and stems increased from May to February in all cultivars. The role of new and old leaves in N storage on evergreen cultivars varied with cultivar and time. Differences in N storage between the evergreen cultivars occurred primarily in their roots and leaves. Over the winter, PJM stored more N in its roots, whereas ER stored more N in its leaves. Changes in N concentrations and contents in different plant structures after November indicate that, during early winter, N stored in other structures moves to roots and old stems of PJM, old stems of ER, and roots and new and old stems of AZ. These results suggest that fertilizer application strategies for transplanted liners of these cultivars should include low N availability after transplanting followed by high N availability in mid to late summer. This type of strategy will not only improve N uptake efficiency from fertilizer, but also will minimize N loss from the containers. The results also demonstrated that N uptake in the autumn may play an important role in supplementing plant N reserves required for growth during the next season as well as for balancing N losses incited by leaf abscission, root turnover, and maintenance functions that occur over winter.