In the field a wide variety of both annual and perennial weeds may occur together. The horticulturalist must recognize the most important weeds in their holding or garden, so that a decision on the precise use of chemical control with the correct herbicide is achieved. Particular care is required to match the concentration of the herbicide to the weed species present. Also, the grower must be aware that continued use of one chemical may induce a change in weed species, some of which may be tolerant to that chemical.
These simple plants may become weeds in wet growing conditions. The small cushion-forming moss (Bryum spp.) grows on sand capillary benches and thin, acid turf that has been closely mown. Feathery moss
(Hypnum spp.) is common on less closely mown, unscarified turf. A third type (Polytrichum spp.), erect and with a rosette of leaves, is found in dry acid conditions around golf greens. Liverworts (Pellia spp.) are recognized by their flat (thallus) leaves growing on the surface of pot plant compost (see Figure 13.16).
These organisms increase only when the soil and compost surface is excessively wet, or when nutrients are so low as to limit plant growth. Cultural methods such as improved drainage, aeration, liming, application of fertilizer and removal of shade usually achieve good results in turf. Control with contact scorching chemicals, e.g. alkaline ferrous sulphate, may give temporary results. Moss on sand benches becomes less of a problem if the sand is regularly washed.
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