In reply to JDal:
The original act has been amended (
http://www.legislation.gov.uk/ukpga/1981/69 ) and 'recklessly' has now been added, I have pasted the relevant parts below. I am not a lawyer so cannot give authoritative guidance and do not intend to speculate wildly (as is so often the case on UKC) I think it wise however that those intending to do some 'gardening' on routes be aware of the potential legal implications and for the sake of any rare and endangered species they should take the trouble to identify plants before simply destroying them. There are many crags where access for climbers is a sensitive issue, we share the places we climb with others and if we do not take care of those places properly we run the risk of damaging our own cause.
13 Protection of wild plants.
(1) Subject to the provisions of this Part, if any person—
(a) intentionally or recklessly picks, uproots or destroys
(i) any wild plant included in Schedule 8; or
(ii) any seed or spore attached to any such wild plant; or
(b) not being an authorised person, intentionally or recklessly uproots any wild plant not included in that Schedule, he shall be guilty of an offence.
(2)...
(3) Notwithstanding anything in subsection (1), a person shall not be guilty of an offence by reason of any act made unlawful by that subsection (“an unlawful act”) if he shows—
(a) that the unlawful act was the incidental result of a lawful operation or other activity;
(b) that the person who carried out the lawful operation or other activity—
(i) took reasonable precautions for the purpose of avoiding carrying out the unlawful act; or
(ii) did not foresee, and could not reasonably have foreseen, that the unlawful act would be an incidental result of the carrying out of the lawful operation or other activity; and
(c) that the person who carried out the unlawful act took, immediately upon the consequence of that act becoming apparent, such steps as were reasonably practicable in the circumstances to minimise the damage to the wild plant in relation to which the unlawful act was carried out.