Hi Sven,
The effect of section 4(3) is, in general to exempt things growing wild from the law of theft. It will be theft however if:
(a). (except in the case of mushrooms) the defendant removes the whole plant. For example, he pulls out a primrose or a sapling by the roots. This is not picking from a plant and so is not within the exception.
(b), The defendant removes the plant or part of t by an act which cannot be described as 'picking'. For example, he saws off the top of a Christmas tree growing wild on the Plaintiff's land, or cuts the grass growing wild of the Plaintiff's land with a reaper or a scythe.
(c) The defendant picks mushrooms or wild flowers, fruit or foliage, for a commercial purpose - for example, mushrooms for sale in his shop or holly to sell door to door at Christmas. The provision is (according to JC Smith: Law of Theft) intended to be used against depredation on a fairly large scale but t would seem to cover such cases as where the defendant, a schoolboy, picks mushrooms intending to sell them to his mother or the neighbours. According to Smith, it is possible, however, that such a single isolated case might be held not to fall within the law as not being a 'commercial' purpose - for it will be noted that the wording of the subsection requires that the defendant, to some extent, must be making a business of dealing in the things in question.
It will, of course be theft to pick a single cultivated flower, wherever it is growing.
As for the tree, the issue of whether it is cultivated or wild is a question of fact and whether it was reasonably discoverable.
However, the law of theft has a mental element to it which must be satisfied for a prosecution to succeed. Theft requires dishonesty to be proved and the test of honesty is a two-stage objective/subjective test set out in the case of R v Ghosh [1982] 1 QB 1053. the first stage is to ask:"was the defendant dishonest according to the stands of ordinary decent people? (the objective part). If the answer is yes, then the second stage of the test, (the subjective part) is to ask did the defendasnt reaslise that what he was doing was dishonest by those standards?
(edited to add the Ghosh test of honesty)
Regards and best wishes
Iolis