Strange solving process for this one, as I started at the bottom and worked my way up. On a first reading of clues I could not get a single one in the top half of the grid, and I think the bottom half was complete before any of the entries poking up into the top half gave me a fingerhold.

Overall I found this a touch more difficult than recent Mephisto puzzles.

Definitions can be confirmed in Chambers and so I will focus on the wordplay in this report.

Away we go...

Across
1	Plants are having stems, therefore needing to be pinned (10)
	ACANTHUSES - A(are, unit of measurement), then CANES(stems) containing THUS(therefore)
10	Maybe like organ passage with tweaking of a gut (6)
	FUGATO - anagram of OF,A,GUT
12	The French place, a strange island (6)
	LAPUTA - LA(the in French), PUT(plac), A. Strange due to its connection to Gulliver's Travels
13	Severe old people affected somehow by contact with insect (11)
	STERNOTRIBE - STERN(sever), O(old), TRIBE(people)
15	Article about terrible ruler very unacceptable in a Greek location (6)
	AONIAN - A(article), ON(about) and then IVAN the terrible missing V(very)
16	What's odd in old company, looking back? Shut up! (6)
	IMMURE - RUM(odd) in EMI(old musical company) all reversed
17	Stopping short, come to river or lake (4)
	MEER - shorten MEET(come to) then R(river)
18	Weed became tainted (6)
	STALED - double definition, the first being relatied to cattle urination
19	Advice before putting back sheets or mattress? (6)
	AIRBED -  before you put back the sheets you should AIR the BED
20	Embrace one getting cross, needing to gather support (6)
	ABRAZO - A(one), then ZO(cross) containing BRA(support)
24	Gravedigger’s therefore getting it wrong — one wild creature? (6)
	ARGALI - if you check in Chambers, ARGAL is a bungled attempt by a gravedigger in Hamley to say ERGO(therefore), then I(one)
28	Act involving three thousand outside university (4)
	MUMM - M,M,M (three thousand) surrounding U(university)
29	Need, say, to get from ’ere (6)
	EGENCE - EG(say) then get from 'ENCE
30	Juniper to preserve outside home (6)
	SAVINE - SAVE(preserve) containing IN(home)
31	Rash I sorted out in medical treatment (11)
	ARTHRODESIS - anagram of RASH,I,SORTED
32	Having less rain than anywhere else, stride out (6)
	DRIEST - anagram of STRIDE
33	Insist on providing very small amount for beggar? (6)
	THREEP - the beggar in The Threepenny Opera would want THREE P
34	When in trouble, rangers eat ants (10)
	ERGATANERS - anagram of RANGERS,EAT

Down
2	Copper to advertise safety device (6)
	CUT-OUT - CU(copper), TOUT(advertise)
3	Particular opening’s marked by song in the Netherlands (6)
	NARIAL -  ARIA(song) in NL(the Netherlands)
4	Got hold of by Bob in Perth, I acted like a lout (6)
	HOONED -  HOD(bob, Scottish) containing ONE(I)
5	Superior officer meets foreign friend turning up in den in garden? (6)
	ULTIMA -  U(superior), LT(lietenant, officer), then AMI(foreign friend). The definition is that DEN is the ultima of GARDEN
6	Use this chemical compound next in novel experiment (6)
	EPIMER - anagram of EXPERIMENT minus NEXT
7	Bring new development to old city and don’t allow one to have buses running round (11)
	SUBURBANISE - UR(old city) and BAN(don't allow) I(one) inside an anagram of BUSES
8	What’s harvested sadly had to be ditched — go very hungry, as before (6)
	STERVE - anagram of HARVESTED minus HAD
9	German chemist has a meal — fish (10)
	HABERDINES - Fritz HABER(German chemist) then DINES(has a meal)
11	Foreign officers with inside information meeting challenge outside club (11)
	GENDARMERIE - GEN(inside information) then DARE(challenge) surrounding MERI(a kiwi club)
13	Serve salad spiced up with sauce (10, two words)
	SALSA VERDE - anagram of SERVE, SALAD
14	Head giving audible refusals (4)
	NAZE - sounds like NAYS
21	Graffiti in the gents showing actor (6)
	BOGART - graffiti in the gents could be BOG ART
22	East End ghost maybe in old-style escapade (6)
	AUNTER - remove the aspirant from HAUNTER(ghost)
23	Thanks after measure is laid down in code of honour (6)
	OMERTA - TA(thanks) after something that sounds like HOMER(a Hebrew measure)
24	Idiot getting too much infatuated, as of old (6)
	ASSOTT - ASS(idiot), OTT(too much)
25	Attack has a couple of characters turning up in Moroccan house (4)
	RIAD - jumble the middle characters of RAID(attack)
26	Good knight embracing maiden possibly gets control (6)
	GOVERN - G(good), N(knight) containing a maiden OVER
27	Having more rows maybe in boat around island (6)
	LINIER - LINER(boat) surrounding I(island)

23.32. I’m on holiday this week, so will keep it brief. I found this puzzle quite hard, but a lot of fun to solve.

Definitions are underlined, anagrams indicated like (TIHS)*, anagram indicators are in italics.

Across
1	Gelding coming from shed to allotment
	CASTRATION - CAST, RATION. A snake can be said to CAST its skin.
6	Short period without a female companion
	STAG - STAGe.
9	A composer’s decision to avoid court
	VERDI - VERDIct.
10	In video, a Sixties film originally has become clearer
	TAKE SHAPE - T(A, KES, Has)APE. I struggled a bit with this one: I could see TAKE SHAPE fitted the definition but it took me ages to see how the wordplay worked.
12	Meet with us and get hammered?
	WEST HAM UNITED - (MEET WITH US AND)*. &Lit. West Ham are known as the Hammers.
14	A lot is happening to garden tools
	IN SPADES - IN (trendy, happening), SPADES.
15	Country vegetable, never ever thrown away
	SWEDEN - SWEDE (or rutabaga as it’s known in North America), Never. Another where the wordplay eluded me for a while.
17	Execution site brought back large stake
	GIBBET - reversal of BIG, BET.
19	Appearances of rock groups put on abroad
	OUTCROPS - OUT (abroad), CROPS. I hesitated over this because I thought ‘groups’ was a bit loose for CROPS, but it’s there in expressions like ‘the current crop of politicians is profoundly depressing.’
21	Eco-friendly rabble I goaded to riot
	BIODEGRADABLE - (RABBLE I GOADED)*.
24	Prisoner’s honest, witness finally explains
	CONSTRUES - CON’S, TRUE, witnesS.
25	Where mourners might be attentive
	AWAKE - or A WAKE.
26	Spring arrest
	STEM - DD.
27	Wrong about herb with duck after mushroom turnover
	PECCADILLO - CA (about), DILL, O after a reversal of CEP (mushroom).

Down
1	Hoover up a drink
	CAVA - reversal of VAC, A.
2	Goes slowly after son carelessly dashes off
	SCRAWLS - S, CRAWLS.
3	Recalling testimony, under control
	REINSTATEMENT - REIN (control), STATEMENT (testimony). A football player might be recalled/reinstated to a team after being dropped.
4	Row about high tech becomes more peevish
	TETCHIER - TIER (row) containing (TECH)*.
5	Ropy stuff for sealing wood, I’m thinking
	OAKUM - OAK, UM. ‘Loose fibre obtained by untwisting old rope, used especially in caulking wooden ships’, ‘caulk’ being a ‘waterproof filler and sealant, used in building work and repairs.’ A rather obscure word I have learned from crosswords.
7	How soldiers might be saluted
	TOASTED - the secondary definition refers to the kind of soldiers you dip in a boiled egg.
8	Rocky edge with Sirens being rapacious
	GREEDINESS - (EDGE SIRENS)*.
11	Ads for carrier go on to support submarine?
	SANDWICH BOARD - BOARD (go on) underneath SANDWICH, of which a submarine or sub is a variety.
13	On foot, little one promotes rides from friends
	PIGGYBACKS - PIGGY (the little toe), BACKS (promotes).
16	Wrongly accused Liberal’s entered Downing Street perhaps
	CUL-DE-SAC - L contained in (ACCUSED)*.
18	Cheese stores have cake
	BROWNIE - BR(OWN)IE.
20	More than a couple of lines in total
	OVERALL - OVER (more than) A, LL.
22	After defeat, evil leader gives way
	ROUTE - ROUT, Evil.
23	Turn down a little love token
	VETO - contained in ‘love token’.