Random House unabridged:
sear¹ - ... (adj.) 8. sere¹.
sere¹ - (adj.) dry; withered. Also, 'sear.'

Webster's 3rd unabridged:
sere¹ /also/ sear - 1: dried up: WITHERED <rank summer vegetation turns ~ —Marjorie K. Rawlings> <~, cracked mud flats ...> 2: [archaic] worn thin: THREADBARE <sails that were so thin and ~ —S.T. Coleridge>

NOAD:
sere¹ - (adj.) variant spelling of SEAR.
sear - ... (adj.) (also 'sere') [poetic/literary] (esp. of plants) withered

Sere--occasionally spelled sear--is one of our more ancient words; it is chiefly an adjective meaning 'dry' or 'withered', and is often (though not always) used of plants or places where one might find them. ... Examples: "He is deformed, crooked, old, and sere" (Shakespeare, Comedy of Errors); "Now here, now there, an acorn, from its cup / Dislodged, through sere leaves rustled" (Wordsworth, The Prelude); "... a bee bustling/Down in the blue-bells, or a wren light rustling / Among sere leaves and twigs, might all be heard" (Keats, Endymion).
As these examples suggest, the word sere certainly does sound high-flown and poetic, and some dictionaries go so far as to label it "poetic" or even "old poetic." There is no question, though, that the word is still in use in nonpoetic contexts. ... "It was a hot September morning, all of Fresno and the broad dusty valley beyond held in the grip of something stupendous, a blast of air so sere and scorching you would have thought the whole world was a pizza oven with the door open wide" (T. Coraghessan Boyle, granted, a writer obsessed with obscure words, in The New Yorker); "Every year the state [sc. California] moves 14 trillion gallons of water ... fitting the sere landscape with a caul of pipes, ditches, and siphons that irrigates an agricultural empire" (Harper's).
... The word sear 'to burn or char; scorch', also found in Old English, is actually derived from sere (which, as noted, is sometimes spelled sear).
http://www.randomhouse.com/wotd/index.pperl?d...

• Through sere trees and beheaded grasses the slow rain falls (Jane Kenyon, 'August Rain, After Haying')
• The leaves they were withering and sere ... But our thoughts they were palsied and sere (Poe, 'Ulalume')
• Thomas Hardy, 'Wives in the Sere'
• A Field of Stubble, lying sere / Beneath the second Sun (Emily Dickinson)
• MY life -- to Discontent a prey -- / Is in the sere and yellow leaf. (Henry S. Leigh)
• But there be who walk beside / Autumn's, till they all have died, / And who lend a patient ear / To low notes from branches sere
(Walter Savage Landor, 'Very true, the linnets sing')
• Dear child, within each sere dead form There sleeps a living flower (Harriet Beecher Stowe, 'The Crocus')

The normal, correct spelling is 'sere'; if 'sear' still exists as a variant at all, it's surely much less common. (I think the NOAD was out of its mind to list 'sear' first; they must have counted typos or something.)

The [bot.] marking is too narrow, as it's not a technical botanical term but rather a literary adjective, used in elevated diction. It may not be poetic in the sense of used only in poems, but most people who know it probably do know it from poetry, and unlike the Random House editors, I would indeed call it literary. People do occasionally use literary words in novels and newspapers, but that doesn't make the words ordinary. Since LEO doesn't have a marking for literary, I think [poet.] is about the closest, and will be much more helpful to users than no marking at all; but if not that, then at least definitely [form.].

I don't know what the exact differences are between vertrocknet, ausgetrocknet, ausgedörrt, welk, etc., so I'll leave the German side to others.