of these act by interfering with the growth of the weed and are often based on plant hormones. Herbicides are generally classified as follows: Contact herbicides destroy only plant tissue that contacts the herbicide. Generally, these are the fastest-acting herbicides. They are ineffective on perennial plants that can re-grow from roots or tubers. Systemic herbicides are foliar-applied and move through the plant where they destroy a greater amount of tissue. Glyphosate is currently the most used systemic herbicide. Soil-borne herbicides are applied to the soil and are taken up by the roots of the target plant. Pre-emergent herbicides are applied to the soil and prevent germination or early growth of weed seeds. In agriculture large scale and systematic procedures are usually required, often by machines, such as large liquid herbicide 'floater' sprayers, or aerial application. These are thought to likely have several substantial detrimental impacts (e.g. on soils, health and insects) – which may partly explain the development of alternatives described here – and there are also systematic procedures using herbicides that have lower impacts such as robots and machines that apply low amounts with high precision. Organic approaches Organic weed control involves anything other than applying manufactured chemicals. Typically a combination of methods are used to achieve satisfactory control. Sulfur in some circumstances is accepted within British Soil Association standards. Bradley method The Bradley Method of Bush Regeneration uses ecological processes to do much of the work. Perennial weeds also propagate by seeding; the airborne seed of the dandelion and the rose-bay willow herb parachute far and wide. Dandelion and dock also put down deep tap roots, which, although they do not spread underground, are able to regrow from any remaining piece left in the ground. Hybrid One method of maintaining the effectiveness of individual strategies is to combine them with others that work in complete different ways. Thus seed targeting has been combined with herbicides. In Australia seed management has been effectively combined with trifluralin and clethodim. Herbicide resistance Resistance occurs when a target plant species does not respond to a chemical that previously used to control it. It has been argued that over-reliance on herbicides along with the absence of any preventive or other cultural practices resulted in the evolution and spread of herbicide-resistant weeds. Increasing number of herbicide resistance weeds around the world has led to warnings on reducing frequent use of herbicides with the same