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Pecan [Carya illinoinensis (Wangenh.) K. Koch] growers are advised to control orchard floor vegetation when establishing new orchards, but there is not a set recommendation for vegetation control in mature orchards. The objective of this study was to measure the effect of orchard floor vegetation on water and nitrogen (N) status of flood-irrigated mature pecan trees. Four treatments studied were: completely vegetated orchard floor, vegetation-free inner area directly under the tree canopy with vegetation in the outer area, completely vegetation-free, and vegetated inner area under the canopy with a vegetation-free outer area. Treatments were organized as a 2 × 2 factorial structure with inner and outer treatment factors, both with levels vegetated and vegetation-free. Soil moisture and tree midday stem water potential (MSWP) were measured during irrigation cycles to evaluate the development of water stress in the pecan trees. Soil moisture data showed a significant outer main effect when the soil in the entire orchard was the driest, that is, just before irrigation events. Areas with vegetation cover that were exposed to full sun were significantly drier than shaded vegetated areas and vegetation-free areas in the orchard floor. However, this was not correlated with differences in tree water status as indicated by MSWP. Leaf tissue and soil analyses showed no significant differences in N concentrations among treatments in either year. Treatments with orchard floor vegetation in the outer area had significantly higher yield efficiency and marginally significant improvements in percent kernel fill and number of nuts per kilogram. Our findings suggest that there may be more benefits to maintaining orchard floor vegetation in mature orchards than were previously acknowledged.

Salinity responses and salinity-related suppression of budbreak of drip-irrigated pecan [Carya illinoinensis (Wangenh.) K. Koch] seedlings under different irrigation water salinity (EC_IRR) levels were investigated in the pot-in-pot system. The 1-year-old pecan seedlings of rootstock ‘Riverside’ grafted with ‘Western Schley’ scions were transplanted in pots filled with sandy loam soil and grown for 2 years under the same amount of irrigation water but four irrigation EC_IRR treatment levels consisting of 1.4 dS·m⁻¹ (control), and three qualities of irrigation water obtained by using a solution of calcium chloride (CaCl₂) and sodium chloride (NaCl) in a ratio of 2:1 (by weight) to reach the EC_IRR levels of 3.5, 5.5, and 7.5 dS·m⁻¹, respectively. The leachate electrical conductivity (EC_d) was highly correlated with soil salinity (EC_1:1) and was significantly higher when the irrigation EC_IRR treatment levels increased from 1.4 (control) to 7.5 dS·m⁻¹. However, both EC_d and EC_1:1 remained nearly constant within the same irrigation EC_IRR treatment level during both years. Increasing salinity in irrigation water, particularly the EC_IRR levels of 5.5 and 7.5 dS·m⁻¹, showed significantly low seedling height and stem diameter growth and delayed or even inhibited budbreak in the seedlings. The EC_1:1 that inhibited seedling heights, stem diameters, and budbreak was somewhere between 0.89 and 2.71 dS·m⁻¹ (or EC_IRR between 1.4 and 3.5 dS·m⁻¹ and EC_d between 2.10 and 4.86 dS·m⁻¹), providing that soil water content was not a limiting factor in the root zone and irrigation water was uniformly distributed in the confined root zone to obtain uniform salt leaching. The visual symptoms of leaf scorch for irrigation EC_IRR levels of 3.5, 5.5, and 7.5 dS·m⁻¹ also indicated that somewhere between 0.89 and 2.71 dS·m⁻¹ of the EC_1:1, salt injury started to occur. Increasing salinity in irrigation water significantly increased chloride (Cl^–) accumulation but reduced nitrogen (N) content in the scorched leaves, particularly under the irrigation EC_IRR levels of 5.5 and 7.5 dS·m⁻¹. Leaf scorch symptoms in pecan seedlings were likely associated with Cl^– toxicity. No pecan seedlings under the irrigation EC_IRR treatment levels of 5.5 and 7.5 dS·m⁻¹ survived to the end of the 2-year growing period. Thus, threshold EC_1:1 was somewhere between 0.89 and 2.71 dS·m⁻¹ beyond which plant injury increases with increasing EC_1:1 threatening the survival of pecan seedlings.