The effect of environmental factors and crop interference on the biology of yarrow (Achillea millefolium L.) seed and seedlings

Abstract

A population of 58 seedling yarrow (Achillea millefolium) plants m⁻² produced approximately 243,000 viable seeds per square metre in the first season of growth and development. Approximately 10% of the freshly harvested yarrow seed had no dormancy and was able to germinate in the presence of adequate amounts of water and ambient conditions of temperature and aeration suitable for normal vegetative growth and development. The rest of the imbibed seed required direct light for complete germination. Even in the absence of light, 30% to 50% of these dormant seeds germinated when scarified and/or stratified or when supplied with 10⁻³M KNO₃ solution or when subjected to diurnal alternation of temperature of 20º - 30ºC. It is evident that at least five conditionally dormant types of seed are present in yarrow.

When adequate moisture was available the yarrow seed lying on the soil surface germinated and established seedling plants in the spring, summer and autumn months. However, when the seeds were buried in the soil a substantial proportion of them did not germinate due to the lack of the special environmental cues required to break dormancy. They remained viable for varying lengths of time depending on the depth at which they were buried in the soil profile. Approximately 50% to 60% of the seed buried at 16 cm and 32 cm, respectively, remained viable after 2 years while only ≤ 10% of the seed buried at 8 cm or less were viable after the same period of time. The viable seed germinated when subsequently exposed to light.

It is suggested that the above detailed characteristics of the yarrow seeds are of ecological importance as they would undoubtedly ensure that the seeds germinate close to the soil surface in land relatively free of other vegetation and the ambient conditions present at the time of seed germination would be suitable for the normal growth and development of the emerging seedlings.

The reduction of light availability to seedling yarrow plants markedly suppressed their growth and development and indicated that it was essentially an obligatory 'sun' species. However, established yarrow seedlings survived in 6.4% daylight and their total reproductive effort, at this light intensity, was directed towards rhizome production. Similar trends in vegetative growth and development and the reproductive effort were observed when seedling yarrow stands were grown in association with barley (Hordeum vulgare) or pea (Pisum sativum) crops. Although the aggressivity of seedling yarrow grown with barley or pea plants was low during the early phase of vegetative growth and development, the yarrow in association with the pea plants exhibited markedly better growth and development compared to when grown with the barley plants. Both crop species shaded the yarrow plants grown with them and also obtained a greater share of the available soil N, P and K. In addition to this, the barley roots appeared to exert an allelopathic influence on the neighbouring yarrow plants which was deleterious to yarrow growth. The greater penetration of light through the pea canopy and the absence of allelopathic interference by the pea plants were important factors contributing to the comparatively better growth and development of seedling yarrow plants associated with this crop species.

When seedling yarrow plants were grown with barley or pea plants, root interference between them commenced earlier than shoot interference. In the yarrow/barley association root interference continued to be of greater importance than shoot interference in suppressing the growth of the former species during the early stages of vegetative growth. The converse was true in the yarrow/pea plant association.

The yarrow plants present in the barley or pea crops grew rapidly once the crops were harvested with rhizome development being a major contributor to the increased growth.

The characteristics of the yarrow seeds and seedlings helped explain the persistence of the species in arable land while the markedly better growth and development of the yarrow seedlings in association with the pea crop showed that it was an opportunistic weed. The usefulness of the current knowledge of the biology of yarrow in planning and executing various mechanical and cultural practices aimed at controlling the species on arable land are detailed and areas of further study are suggested.