Sometimes mould fungi grow over weathered coated sapwood, whereas the coated heartwood is not colonised. Two factors may be responsible for these differences: Either the larger content of nutrients in the sapwood, or the content of biocidal extractives in the heartwood. In this study the factors for the difference in mould growth on coated heartwood and sapwood of Pinus sylvestris (Scots Pine) and Gleditsia triacanthos (Honey Locust) were studied. Prior to coating, the samples were leached with water for 14 or 30 days. The coated samples were then stored under favourable conditions for mould growth for 10 weeks. The coating used for the experiments is free of biocides, but shows no mould growth when applied on glass. For G. triacanthos the leaching procedure reduced the growth of mould both on sapwood and on heartwood. Nutrients migrating through the coating are obviously the dominant factor for mould growth on this wood species, whereas water-soluble inhibitory extractives in the heartwood are not relevant. For the sapwood of P. sylvestris the same tendencies were observed. No clear statements are possible for the heartwood of P. sylvestris.

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