It is difficult to measure the costs caused by weeds. An immense amount of time is spent by home gardeners removing weeds from their gardens and lawns. In the agricultural, horticultural or forestry sectors, costs include lost production as well as weed control. An estimate by Monsanto (an agricultural biotechnology corporation) in 1984 put the overall cost of weeds in New Zealand at $393 million, equivalent to $1.03 billion in 2007 terms.
In 2003 it was estimated that giant buttercup (Ranunculus acris) cost the dairy industry $156 million in one year in lost production. Californian thistle (Cirsium arvense) was calculated in 1999 at costing about $10 million annually.
With the climate expected to become warmer in the future, weeds currently restricted to the north of New Zealand will spread further south. Those that prefer a temperate climate may find future conditions less suitable and die off in some areas. Plants like gorse may spread more rampantly into areas that now suffer heavy frosts, such as Central Otago and parts of Hawke’s Bay. Higher incidence of droughts and floods, which lead to increased areas of bare ground, will see greater numbers of annual weeds like thistles., which can thrive in such conditions.
Some weeds have the potential to become serious weeds, but so far have limited distribution, although climate change could hasten their spread. These include several grasses and sedges, Japanese knotweed (Fallopia spp. – already a major problem in Britain), field horsetail (Equisetum arvense – a serious problem in several countries), and a number of aquatic weeds.