Where & Tare by Dipper

It's been a little while since we visited Dipper's garden, but now we see that he has been busy keeping the weeds at bay. As usual with this series, there was little mystery about the theme. Superfluous letters in the wordplay spelt out a series of common weeds -- knotgrass, ground-elder, ragwort, groundsel and nettle - and nipplewort appeared in a main diagonal in the completed grid. But the puzzle made for an entertaining and not too taxing evening's diversion.

Across
4 Being acutely toxaemic, prepare regular doses of utile chalky mepacrine (9)
ECLAMPTIC[K] (Jumble of alternate letters of UTILE CHALKY MEPACRINE)
What made this clue tough, despite the obvious wordplay, was that it used the odd letters of CHALKY but the even letters of UTILE and MEPACRINE,
arguably unfairly. "Regular doses" of a sequence of words ought really to apply to the concatenation of the words.
12 Grandma eating head of nematode - then lied about worm (7)
[N]ANNELID (N (first letter of NEMATODE) inside NAN + jumble LIED)
14 Actors revel in closing words of play about unopened tombs (12)
TRI[O]TAGONISTS (RIOT inside TAG + ON + CISTS minus first letter)
15 Annoys politician in first half of electioneering (5)
HUMPS[T] (MP inside HUSTINGS)
16 Purifiers distilling safer gin without a hint of alcohol (6)
FINERS[G] (Jumble SAFER GIN minus A (first letter of ALCOHOL))
17 Leading edge of rotor, like trailing edge of rudder, connected to wing (4)
[R]ALAR (First letter of ROTOR + A LA + Last letter of RUDDER)
19 Some obstinate fat persists around non-stick coating (4)
PT[A]FE (Hidden, reversed, in obstinatE FAT Persists)
Unannounced abbreviations don't belong as crossword answers, in this solver's opinion.
20 Terrible waste for a couple of Scots (4)
TWAE[S] (Anag. WASTE)
22 Estonian lake containing unknown alcoholic-related substance (5)
E[S]THYL (ESTH (Estonian) + Y (unknown) + L(ake))
24 Respond to great changes around beginning of century (5)
REACT[G] (C(entury) inside jumble GREAT)
26 Notions, in part, insider eschews (5)
IDE[R]ES (Hidden inside insIDER ESchews)
27 Planet easily orbiting furthest points of Pluto and Jupiter (5)
EA[O]RTH (Last letters of PLUTO and JUPITER inside EATH)
28 Part of pedestal discovered in Padua donjon (4)
D[U]ADO (Hidden inside PaDUA DOnjon)
30 Catty expression upsetting women (4)
MEOW[N] (Anag. WOMEN)
33 Act after initially shunning first principle (4)
S[D]EED (First letter of SHUNNING + DEED)
34 Sacred sound, a symbol of the bible (6)
OMAS[E]AL (OM + A + SEAL)
Gregson is always caught out by this meaning of bible.
36 Ignoring singular bond, noble lords reform peers (5)
EORLS[L] (Composite anagram NOBLE LORDS minus S(ingular) BOND)
38 Seven-sided plates I'd later shatter (12)
SEPTILATERAL[D] (Anag. PLATES I'D LATER)
39 Written law confining trade union to built-up area (7)
[E]STATUTE (TU inside ESTATE)
40 Lacking pomp? Lacking money? (9)
STATE[R]LESS (Two meanings)

Down
1 Route right into Maori fort (4)
PA[R]TH (RT inside PAH)
2 Run amok - it sounds like you live without control (7)
UNRULE[A]D (Jumble RUN + U + LEAD)
3 I'm among allied dead endowed with life (8)
A[G]NIMATED (I'M inside AGNATE + D(ead))
5 Embrace rules in Communist Party (5)
CLA[W]SP (LAWS inside CP)
6 Capote character losing head easily (7)
[O]LIGHTLY (GOLIGHTLY minus first letter)
I.e. Holly Golightly, character in Breakfast at Tiffany's.
7 Nationalist in commune that is inventor of rifle (5)
MIN[R]IE (N(ationalist) inside MIR + I.E.)
8 Dogs bury mailman outside (8)
POINTERS[T] (INTER inside POST)
9 Shuffling two eights with wrongly discarded card (4)
TOSE[G] (Composite anagram TWO EIGHTS minus WITH)
10 Effervescent trainer losing energy within (5)
INTRA[R] (Jumble TRAINER minus E)
11 Pampered pupil in very good group. Just the opposite (8)
COS[O]SETED (SO + SET inside COED)
13 Couple - very loud - remove clothing (4)
D[U]OFF (DUO + FF)
17 Repeated entropy in glands initially suggesting blood disorder (8)
ACIDOS[N]IS (DO (=ditto) + S (entropy?) inside ACINI + first letter of SUGGESTING)
18 This provides resistance to threads unravelling (8)
RHEOSTAT[D] (Anag. TO THREADS)
21 Scatter straw around earth - snug for a dog (8, two words)
WATER[S] RUG (E(arth) inside jumble STRAW + RUG)
23 Baby born a long time before mischief-maker (7)
NE[E]ONATE (NE (born) + EON + ATE (God of mischief))
25 Hydrogen in cell consumed haemoglobin-like substance (7)
CHEL[L]ATE (H inside CELL + ATE)
29 Catkin pieces an insect swallows (5)
A[N]MENT (MEN inside ANT)
30 Old coin - one hidden by Zola, for instance (5)
[E]MAILE (A inside EMILE)
31 Measures pig in place of seal (4)
EL[T]LS (ELT + LS (in place of seal))
32 Monstrous 20 stone grasshoppers (5)
WETAS[T] (Jumble TWAE (i.e. 20A) + ST(one))
35 See tropical plant in Samoan city (4)
[L]APIA (PA + PIA)
37 Draw signs backwards (4)
S[E]LED (DELES rev.)