• Nature Book Sample Pages - to help you choose a book
• Nature Forums Search - to help you find a nature forum (of which some botany ones are listed below).
Listed approx North to South (with Natural History Societies in brackets, and Pg for a page rather than a group)
• I recommend the enhanced value of being active in the smaller groups rather than the larger regional ones; there will be experts on both but the smaller ones have a greater sense of closeness.
• Northumberland • (Carlisle) • Cumbria Plants • Cumbria Fungi • N Wales/N England (non-flowering plants) • NE England • Durham • Teesdale • Lancashire/Merseyside • Warrington • NW Yorkshire • NE Yorkshire • North Linconshire (VC54) • South Lincolnshire • Shropshire • Derbyshire/Staffordshire • Nottinghamshire • Northhampton/Derbyshire • East Anglia (1) • East Anglia (2) • Brecklands (N&E Anglia) • Bedfordshire • (Bristol) • Worcester (Pg) • Somerset • (London) • London (Fungi) • (Essex) • Surrey • Surrey (Orchids) • Kent • Kent (Orchids) • Kent (Fungi) • (Orpington) • Sussex • Sussex (Orchids) • Sussex (Fungi) • Hampshire • (Isle of Wight) • (Cornwall) • Isles of Scilly • Guernsey •
• Wildflowers of Britain and Ireland
• Botanical Discussion (Britain and Ireland) - NB for experienced botanists: you will need a text-only work to answer the entry question, which is "What are the first four words on the page of Stace ed 4 p843, OR Stace ed 3 p126, OR Sell & Murrell vol 2 p108, OR Tutin-Clapham ed 3 p254, OR the names of the 4 segments in the banner picture?" If there's a difficulty seeing the question there, please do PM your answer!
• BSBI Botanical Society of Britain & Ireland (see also • BSBI Ireland • BSBI Scotland)
• Europe • France • Other Countries
Hover over an item for its full title. Many of these groups are seasonal or spring into action some years more than others; but always feel free to post queries in them at any time as most of them have much helpful expertise to give -
• Art • Aquatic Plants • Brambles (Rubus) • Conifers • Cotoneasters • Daffodils (Narcissus) • Dandelions (Taraxacum) • Elms (Ulmus) • Euphrasia • Fumitories (Fumaria) • Galls (& see book) • Grasses/Sedges/Rushes/Ferns/Mosses/Lichens/(Fungi) • Hawkweeds • Orchids 1 • Orchids 2 • Production Group • Roses (Rosa) • Seaweeds/Algae • Sedges (Carex) • Slime Moulds • Trees 1 • Trees 2 • Trees (High-Altitude Finds) • Willows/Poplars (Salix/Populus) • Winter Twig and Bud Recognition
• Also : Hints Group (Deliberately doesn’t give ids, but helps you find the id)
• Mushroom Groups : • A • B • C • D • E
• Gardening Groups (British Isles) : • Identification • Help & Tips
• Botany in Scotland - has a very regular "Plant of the Week"
• Wild Flowers in Cleveland and North Yorkshire
• "Violets and Others" - by Peter Leonard
• 
NE England (James Commons) - click one of the Take a Look links
• Hampshire - click one of the green rectangular links
• Britain & Ireland (David Merrick mostly) - various genera (see its menu)
• 
Cumbria - Click the "Look out for" links
• East Anglia - Some broken links but numerous excellent guides
• BSBI
• Sedges (Carex etc) - by Mark Gurney (alternative location)
• Fumitories - by Mark Gurney
• Conifers in the British Isles (1972, Mitchell) - Superb
• A handbook of Coniferae, including Ginkgoaceæ (1923, ed 1, Dallimore & Jackson) - superb
• 1996 Edition of Blamey & Fitter - can be borrowed for free in 1 hr slots (free login)
NB: Every site has its shortcomings. Some of these links are google searches, and you can replace in the search the plant you are after.