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The most common quality defect is an excessive amount of various foreign substances such as green compost, chopped wood residues, plastic parts, stones and even broken glass. The uniform grain size of the bark mulch is also a quality feature: Although there are different materials depending on the intended use, however, the size of the chunks must be within a certain range. Suppliers of cheap bark mulch usually do without sifting, which is why the products generally contain both large bark pieces and fine material.
In addition to the visually recognizable defects, some can only be detected with laboratory methods. For example, germ tests show whether a bark mulch is plant compatible. Also insecticide residues are an important criterion - especially if the bark comes from abroad. There, the bark beetles in forestry are often still fought with old, hardly biodegradable preparations that have not been approved in Germany for a long time.
A major reason for the poor quality of many bark mulch products is that the raw material - the softwood bark - is becoming increasingly scarce, because it is increasingly being used to generate energy. However, reputable suppliers usually have long-term supply contracts with the forestry industry, which continues to ensure good quality.
In addition, the product name "bark mulch" is not legally defined: The legislature does not stipulate that bark mulch must consist exclusively of bark, and also does not specify any limit values for the proportion of foreign matter. It is also a natural product that inevitably varies in appearance and quality.
For the reasons mentioned, gardening enthusiasts should only buy bark mulch with the RAL seal of quality. The quality requirements have been formulated by the Gütegemeinschaft Pflanzen für Pflanzen (GGS) and must be checked and verified by the manufacturers on an ongoing basis. Because of the elaborate quality assurance, the low-priced bidders largely do without, Rindenmulch with RAL seal in the specialized trade is of course correspondingly more expensive.