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Non-turf ground-covers occupy a significant portion of the landscape, and understanding their water requirements is important when water conservationism being practiced. Six groundcover species (Baccharis pilularis `Twin Peaks', Drosanthemum hispidum, Vinca major Gazania hybrid, Potentilla tabernaemontani and Hedera helix `Needlepoint') representing a range of observed water needs were evaluated under different levels of irrigation based on percentages of real-time reference evapotranspiration.
Treatments of 100%, 75%, 50% and 25% of ETO were applied during 1989 while treatments of 50%, 40%, 30% and 20% of ETO were applied during 1990. Plant performance ratings in the first year indicated that 50% of ETO was the minimum treatment which resulted in acceptable plan aesthetics for all species except for Drosanthemum which performed equally well at each treatment. Significant differences in performance did occur among and within species at the different treatments. Results from 1990 will reveal which species might maintain aesthetic appearance at irrigation levels between 50% and 20% of ETO. These results will be presented and discussed in terms of their significance to species selection and total landscape irrigation management.
Forty eight California sycamores (Platanus racemosa) were planted (5/91) from one gallon containers and mulched (8/91) with, pine bark, composted sewage sludge and wood products, fresh Eucalyptus cladocalyx chips (large 2-6cm), fresh eucalyptus chips (small <1cm), composted large eucalyptus and untreated. Mulches were applied to a depth of 10cm in a 6m2 area around each tree. Irrigations were based on soil moisture depletion and water content was measured by time domain reflectometry. Mulched trees developed more caliper, lower stomatal resistances and lower trunk temperatures. Soil moisture (top 15cm) was greater under mulched trees. Rooting was evident in the sludge, and composted eucalyptus mulches but absent in the pine bark and fresh eucalyptus mulches.
A previous field study had shown that Baccharis pilularis, `Twin Peaks', Drosanthemum hispidum, Vinca major, Gazania hybrid, Potentilla tabernaemontanii and Hedera helix, `Needlepoint', express no loss in relative aesthetic appearance when irrigated for one season at 50% of reference evapotranspiration (ETo), but three species did not perform acceptably at 25% of ETo. In this study these six species were grown in the field for 16 months under treatments of 50%, 40%, 30% and 20% of real-time ETo to more closely determine their minimum irrigation needs.
Analysis of seasonal plant performance ratings indicates that for Vinca, Gazania and Potentilla there is no significant increase in relative performance when irrigated at more than 30% of ETo. Baccharis, Drosanthemum and Hedera exhibited no significant improvement in performance when irrigated above 20% of ETo. A general decline in aesthetic appearance and performance was observed during the study in Gazania and Potentilla at all treatments, suggesting that their long-term minimum irrigation need may be more than 50% of ETo.