A study of Oxalis and its control

Abstract

The general botany of the genus Oxalis has been reviewed and the species present in New Zealand have been described and illustrated. Observations over the growing period 1958 – 1959 and some controlled experiments enabled the life-history of Oxalis latifolia to be elucidated and described. A dormancy of the bulbs and bulbils, which was initiated growing conditions, such as cold temperatures and lack of moisture and nutrients, as discovered and described. A review of the present methods of control of O. latifolia and O. pes caprae has been made and experiments were laid down to test the effectiveness of various soil sterilants on O. latifolia. A new material, Vapam (Sodium N-Methyl di thicarbamate) was found to be almost 100% effective against this species and maintained the soil in a sterile condition for approximately nine days. The effect of leaching and addition of compost, on a soil rendered sterile by sodium chlorate and borax, was investigated. Large quantities of water quickly leached both chemicals from the soil, which subsequently supported the growth of cress (Lepidium sativum). In comparison, addition of compost reduced sterility only slightly. The microbiological life of the soil was not noticeable reduced, but the composition was altered slightly, on addition of chlorate to the soil.

The effect of plant regulators on O. latifolia was investigated in detail. 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2-methly, 4-chloro-phenoxyacetic acid and 2-methly, 4-chloro-phenoxybutyric acid (MCPB) were compared in their effects upon Oxalis. 2,4,5-T gave up to a 45% kill of bulbs while 2,4-D was slightly less effective. MCPA and MCPB affected leaves but were generally ineffective against the bulbs. The basal portion of the bulbs was mostly readily affected, recovery being from the central apex or from bulbils attached to the parent bulbs.

1000 p.p.m. was the most effective concentration used, two sprays, however, at 500 p.p.m. and separated by one week, gave an approximately equivalent leaf kill. The conclusion was reached that the amount entering the plant determined its response, although an excessive concentration in the spray can apparently impair translocation by disrupting leaf tissue.

Spraying in the evening, as contrasted with a morning spray, significantly increased the effect of 2,4-D and 2,4,5-T on Oxalis bulbs, while plants watered with a nutrient solution containing nitrogen, potassium and phosphorus, for three weeks prior to spraying, significantly increased the effect of these plant regulators on Oxalis.

Additives such as sucress, boris acid and urea did not significantly increase or decrease the effect of either 2,4-D or 2,4,5-T on bulbs, but some interesting trends were revealed and discussed.

A study of translocation was made using C¹⁴ labelled 2,4-D applied to the leaves. Autoradiographs indicated at C¹⁴ is moved from the leaves, but does not accumulate to nay great extent in the bulbs and none reaches the untreated leaves within 24 hours. An untreated inflorescence, however, gave a heavy image. Movement thus appears to correspond to movement of carbohydrates in the assimilate stream.

3-amino-1,2,4-triazole (ATA) and maleic hydrazide (MH) did not kill O.latifolia but greatly weakened the plants: time was not available to make full scale investigations with these chemicals.