2018 ASHS Annual Conference

Containerized culinary herbs should be proportional to the container they are produced in so they are aesthetically balanced for marketing and sales. While anti-gibberellin plant growth retardants are commonly used to control containerized-plant growth, they are not labeled for use on containerized herbs and non-chemical growth-control strategies must be used. The objectives of our research were to quantify the impact of restricting irrigation on the growth and tissue nutrient concentration of basil (Ocimum basilicum L.), dill (Anethum graveolens L.), and parsley [Petroselinum crispum (Mill.) Fuss.], and common sage (Salvia officinalis L.). Seedlings were individually transplanted into 4.5-inch diameter round containers filled with a substrate comprised of (by vol.) 75% ground sphagnum peat moss and 25% coarse perlite and amended with 5 lb∙yd^–3 controlled-release fertilizer. Plants were watered to container capacity at transplant, allowed to dry down to volumetric water content (VWC) thresholds of 0.15, 0.23, 0.30, 0.38 or 0.45 m³·m^-3 and subsequently maintained at desired setpoints by using a precision irrigation system controlled by soil moisture sensors. Four weeks after seedlings after seedlings were transplanted and irrigation treatments began, data were collected on plant height and width, node number, leaf area, internode and branch length, chlorophyll fluorescence, photosynthesis, transpiration, and conductance. Shoots were harvested and dried for 3 d, after which mass was recorded. For all four species, plant was positively correlated with substrate VWC. However, magnitude of change in response to VWC varied with species. For example, height of basil increased from 24.6 to 29.2 cm (19% increase) as substrate VWC increased from 0.15 to 0.45, respectively. Alternatively, as substrate VWC increased from 0.15 to 0.45, height of parsley increased from 14.9 to 27.1 cm (82% increase), respectively. Other growth responses followed a similar trend. While conductance and transpiration were unaffected by substrate VWC, photosynthesis of basil and parley increased as substrate VWC increased. Similarly, chlorophyll fluorescence of basil, parsley, and sage were indicative of plants that were not stressed, regardless of substrate VWC. Based on the results of our research, restricting irrigation can suppress growth of containerized herbs. However, the magnitude of growth control varies with species and for some species, such as basil, restricting irrigation may not be sufficient as the sole approach to controlling growth.