Just stumbled on via the HMRC web-site:
http://www.opsi.gov.uk/acts/acts2007/ukpga_20070003_en_53#pt16-ch1-l1g996
Look at section 996; it says:
Meaning of “farming” and related expressions
(1) In the Income Tax Acts “farming” means the occupation of land wholly or mainly for the purposes of husbandry, but does not include market gardening (see subsection (5)).
(2) In subsection (1) “husbandry” includes—
(a) hop growing, and
(b) the breeding and rearing of horses and the grazing of horses in connection with those activities.
(3) For the purposes of the Income Tax Acts the cultivation of short rotation coppice is regarded as husbandry and not as forestry.
(4) In the Income Tax Acts “woodlands” does not include land on which short rotation coppice is cultivated.
(5) In the Income Tax Acts “market gardening” means the occupation of land as a garden or nursery for the purpose of growing produce for sale.
(6) For the purposes of this section “short rotation coppice” means a perennial crop of tree species planted at high density, the stems of which are harvested above ground level at intervals of less than 10 years.
(7) In the application of this section for the purposes of section 192(1) or 303(1) of this Act or paragraph 16 of Schedule 5 to ITEPA 2003—
(a) both references to the occupation of land, and the reference to land on which short rotation coppice is cultivated, refer to land in the United Kingdom, and
(b) the reference to the cultivation of such coppice refers to its cultivation in the United Kingdom.
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HMRC's take on it is here: http://www.hmrc.gov.uk/manuals/bimmanual/BIM55205.htm
"short rotation" is up to 10 years - see above.
This needs careful investigation. It's likely that the law only kicks in once there's income - and the key is "income", not "profit". Once the law applies fold have to tell HMRC by 5th October following the tax year in which income first arises.