Field horsetail
Equisetaceae - Equisetum arvense

What does it look like?

Field horsetail is an erect, colony-forming, primitive fern-ally that grows up to 80cm high and dies back in winter.  The plants produce two types of stems.  Fertile stems appear in spring and die back in summer.  They are whitish to light-brown, unbranched, hollow and leafless.  At the tip of each fertile stem there is a small, yellowish-brown cone.  The infertile stems resemble miniature pine trees.  All aerial parts die back in winter to a deep, branching root system with round tubers.

Why is it a problem?

Field horsetail has become an aggressive weed in parts of New Zealand.  It can establish in a range of habitats and form extensive colonies which are difficult to kill.  Field horsetail is not tolerant to low levels of light but has some tolerance to drought. The spores of field horsetail are very light and can be dispersed over long distances by wind.  Root fragments and tubers may be moved by machinery, water or in soil.  It is also planted and spread deliberately by people who wish to use it for medicinal purposes