Herbicidal compositions

A solid herbicidal composition is made by mixing a hydrate-forming salt with an aqueous solution of a bipyridylium herbicide in proportions such that the hydrate-forming salt combines with sufficient of the water present to give a solid composition. The hydrate-forming salt may be water-soluble (e.g. calcium citrate, calcium nitrate, potassium alum, potassium dihydrogen phosphate, magnesium nitrate, magnesium sulphate, sodium acetate, sodium orthophosphate, sodium pyrophosphate or sodium sulphate) or water-insoluble (e.g. calcium sulphate). Where a water-soluble hydrate-forming salt is employed, the resultant herbicidal composition may be dissolved in water and applied to weeds in the form of a spray. For this purpose, the herbicidal composition may contain a cationic or non-ionic surface active agent and a corrosion inhibitor (e.g. sodium benzoate, metaborate, molybdate, nitrite or dihydrogen orthophosphate or dipotassium hydrogen orthophosphate). Where a water-insoluble hydrate-forming salt is employed, the resultant herbicidal composition may be introduced into canals or rivers in order to destroy aquatic weeds. For this purpose, the release of the bipyridylium herbicide may be retarded by including in the composition a water-insoluble resin (e.g. alkyd resin, urea/formaldehyde resin, phenol/formaldehyde resin, vinyl chloride polymers or copolymers, polyacrylates, polymethacrylates, polystyrene or epoxy resin) or bitumen. In the examples, the bipyridylium herbicides are selected from 1,1\sv-ethylene-2,2\sv-bipyridylium dichloride or dibromide, 1,1\sv-dimethyl-4,4\sv-bipyridylium dichloride, 1,1\sv-bis-beta-hydroxyethyl-4,4\sv-bipyridylium dichloride, 1,1\sv-bis (dimethylcarbamoylmethyl)-4,4\sv-bipyridylium dichloride, 1,1\sv-bis-carboxymethyl-4,4\sv-bipyridylium dichloride 1,1\sv-bis (diethylcarbamoylmethyl)-4,4\sv-bipyridylium dichloride and 1,1\sv-bis(dimethylmorpholinocarbonylmethyl) - 4,4\sv - bipyridylium di - chloride.