Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)




Neil E. West


Dyers woad (Isatis tinctoria L.) is a noxious weed on northern Utah rangelands. Chemical and mechanical means of control are unsuitable for rangelands. A potential alternative, biological control, is difficult due to a lack of basic ecological information. To remedy this, some aspects of the population biology and autecolgy of dyers woad were studied. The feasibility of controlling dyers woad by early spring grazing was also investigated via clipping experiments. A population study followed the survivorship of experimentally established populations over two years. Fall germinating individuals (1984) overwintered as rosettes twice and all survivors reproduced successfully. Spring germinating individuals (1985) overwintered as rosettes only once and 87% seeded the subsequent spring. Thus, on a good condition {high seral) foothill rangeland, dyers woad behaved predominantly as a biennial. Peak mortality in both fall and spring populations coincided with summer drought. The population size of dyers woad was constricted at two stages: (i) germination and establishment, and (ii) young rosette. The risk of mortality mortality in young rosettes was 77%. Dyers woad should be targeted for biological control at this vulnerable stage of growth. The seed dispersal pattern of dyers woad was best described by a negative exponential model of the type logy= a+ bx; (r = .78, a= 1.92, and b = -0.02). Ninety-five percent of a11 fruits were deposited within 54cm of mother plants. The root system of dyers woad was predominantly a taproot with some lateral sin the upper 30cm of the soil. There was little difference between mapped taproot and total mapped root lengths of rosette and mature plants. Significant mortality and reduction in reproductive performance occurred only by clipping, on or after 23 May 1984, at 60% or 90% intensity. Clipping twice, at either intensity, before 23 May had no effect on dyers woad. Sheep utilization of dyers woad ceased after mid-May, and had no significant effect on its mortality, percent flowering, and fruit production. Sheep grazing on dyers woad did not occur when much impact on mortality or seed production could be expected. The stocking re qui red to restrict dyers woad will result in range deterioration. More host-specific biological control agents should be tried.