What tipped the scales in the end was that Crowley simply had one spider plant too many.
To be fair, where Crowley erred was not in quantity but in quality. It had been the better part of a year since he'd had a brown spot or a yellowish leaf to make an example of, and only his suspicion was a match for his pride. On the one hand, while obedience to a higher power wasn't something demons placed much stock in, obedience to him was something Crowley quite approved of, and his were the most obedient plants in the western hemisphere. On the other hand, that was no reason to rest on one's laurels.
Armed with a mister and a stern frown, Crowley stalked from pot to pot, casting a critical eye over glossy, perfect leaves and trembling stems. There should have been at least one plant that didn't quite measure up, but the ferns were positively prehistoric, the ivy was badly in need of a hoary castle to overrun, and only modesty--and a reasonable fear of pruning--kept the elephant ear from outdoing its namesake.
"Hmm," Crowley said, viciously spritzing a philodendron. It shuddered a little, but whether that was from sheer terror or grateful pleasure, he couldn't say. "Well, well," he added, feeling foolish and strangely cheated. It was one thing to make meaningfully ominous comments to keep things running smoothly, but it was Crowley's considered opinion that tossing around threats when there was nothing to complain about was the evil equivalent of buying a big flashy car. Clearly you were a sad little bastard with something to prove.
"Well," he said again, glancing sharply around his flat once more before admitting defeat. "I suppose you all pass," he said grudgingly. "But don't let it go to your heads."
The sudden shivering of leaves didn't perturb him at first. Most of his little chats ended with a profound shaking of foliage. It was probably good for them; knocked the dust off and all. It was only when the ficus grew a foot before his very eyes that he found himself staring, wondering if this was what Aziraphale had meant when the angel said Crowley was bound to be a bad influence on the poor things.
The ficus might have been the first off the mark, but the others weren't slow to catch up. Before Crowley could decide whether to applaud the ficus' motivation or say a few choice words about overachievers, his metaphoric jungle of houseplants was well on its way to becoming a literal one. The ivy--always on the aggressive side--was the first to throw out runners, slithering across the floor right up to Crowley's feet. There was something a little disturbing about the way the vines twitched and curled, rather like a giant squid that wasn't quite certain whether the passenger ship that had just sailed within reach would be too big a mouthful to--
"Gah!" he yelped as the vines, gathering their courage, whipped out boldly to wind their way up his legs. One good yank pulled his feet out from under him, but even as dull stars exploded behind his eyes, it was surprise more than pain that made him hesitate as the back of his head hit the floor. He was a demon, after all. He had options. But as he found himself dragged across the cushioning thickness of his extremely expensive carpet (a carpet as obsessively loyal as its master was obsessively clean, and which wouldn't dream of letting him crack his head open due to a bunch of upstart window accessories), he couldn't help wondering what his plants intended to accomplish. It wasn't like he was growing anything truly dangerous; the dinosaurs had been clever and the platypus was a riot, but he'd always thought the Venus Flytrap was taking the joke too far.
Bemused, he watched stems stretch and leaves spread like the fanning of wings to new, monstrous size. The tugging vines around his legs tightened their grip with bolstered confidence as they gained circumference, grew stout enough to lift him off the floor entirely, and he wondered belatedly if he ought to worry. It wasn't so much that asphyxiation was an unpleasant way to go as that it could be dead embarrassing, and he wasn't exactly in Hell's good books as it was. Getting a new body out of them would be painful enough without fielding smirky questions about what he'd been doing at the time.
"All right," he said sternly as fresh coils wrapped around his arms. "Enough is--"
He'd never known how unpleasant the sound of ripping cloth could be until he'd had a designer shirt shredded off him by a host of clutching vines.
"Hey! Do you know how much that cost?"
The plants weren't listening. They were converging, leaning down toward him in a way that sent a cold spike of panic through him, and never mind that he had occult powers enough to sear everything in his flat to ash with a Word. Prolonged exposure to a demonic aura had clearly given his plants a few powers of their own, but worse than that, the obedience he'd been so secretly pleased with had just as clearly gone out the window. His plants were rebelling, and for one brief moment, he found himself wondering where a vengeful angel was when you needed one.
"Right," he muttered, flexing his hands and finding no slack at all in the vines as his trousers went the way of his shirt. He'd just have to--
He blinked, quite deliberately, and lifted his head a little to look down himself at the long-fronded tendril that was rubbing itself against him through his boxers. Spider plant, he cataloged automatically, jumping a little as the brush of its leaves against the tensed muscles of his stomach left him choking on laughter timed badly with a startled breath.
Thick coils of ivy snaked higher, burrowing into the legs of his boxers and straining against silk. Really, it'd be an excellent time to start fighting back. Change his shape. Try his hand at calling up hellfire. Any moment now.
He hissed when his last few stitches were ripped away, but that was more from the soft, ticklish glide of leaftips over his bared skin than from displeasure. The sharp blades of his hipbones and the thin, sensitive skin just inside their arch were explored with delicate touches that left him writhing until vines and creepers wound him up tight. Nothing to worry about, he told himself at the slow, insistent tugs that spread his limbs wide, left him just barely able to wriggle in their grip. They were just plants, after all. What could they possibly do?
He groaned when a length of vine wrapped itself around his cock--already hard, effort well and truly made for reasons he didn't care to examine too closely. He would have bucked up into that touch for more, but there wasn't enough give in the fibrous coils that held him, and the pleading little jerks of his hips that inched him up into that tight spiral only left him hungry for more. At this rate he'd do something embarrassing, like beg or come off anyway, or...curse long and low, forking tongue stuttering over sibilants as evil-minded ivy slid low and looped, pulled tight around his balls and the base of his prick until he likely wouldn't be coming off until he begged.
"Ngh," he managed, then cleared his throat self-consciously, letting himself go limp in the vines like he didn't have a care in the world. "Not a bad start," he said with an attempt at a normal tone. "I'll give you points for effort. But if you'll put me down now, I'll--"
There was something reaching for him from the middle of the tangle, something green and thick and ridged at the tip.
"Oh, fuck," he said. Though really it was more of a whimper.
He'd forgotten about the Haworthia.
The arms reaching for him were a curious blend of slick and rough, the inner faces smooth as silk, the outer lined with nubby bands that didn't so much scrape as promise as they slid against his calves, the inside of his thighs. The Haworthia had always been hard to read; when he starved it of water until even he began to feel guilty, it went right on being ravishingly green and healthy, and when he finally gave in and drowned it, it would retaliate by shooting up another inch almost overnight, the new growth tender and pale and smooth on all sides. It was the only succulent he owned, and he was more convinced than ever that Aziraphale had had ulterior motives when the angel had given it to him.
"Uh," he said, eyeing the approaching arms--first three, then five--with a dry mouth and sweating palms, oddly torn between apprehension and anticipation. "Look...I don't think that's going to--work--!"
Even at his most distracted, Crowley was no slouch at miracles. It was the work of a moment to convince his body that it didn't need preparation, because the Haworthia wasn't waiting, and the ivy wasn't letting him get a hand in to help things along. There was just the push of a pointed tip, the long, fleshy press of a thick, flaring length as it slid inside, the electric drag of those ridges over his sweet spot, one after the other, making him jerk and gasp with each rub. Oh, fuck, that was good, and he wriggled shamelessly as it sank to its thickest width inside him, all the way to the soft-skinned length where it'd grown too fast to develop its distinctive bands. It was wider than anything he was used to, every breath reminding him of how full he felt, pinned and trapped and split wide.
He groaned when it dragged its way slowly out again, hissed when it surged back in. His spine wanted to come unstrung, his hips rocking mindlessly the spare inches they were allowed, trying to arch up and grind against that last ridge before the whole rippled length was pulled nearly out of him before slamming home. "Oh," he moaned, "that's...that's better, nhhh...."
It wasn't technically begging if there weren't any words to it, but the sound that escaped his throat as he found himself inexplicably empty was distinctly pleading. He didn't have long to wait before he was filled again, but this one felt thicker, its ridges more pronounced and trailing further down its length. When it drove inside him, it was like a string of firecrackers went off all the way up his spine, and if his breath hadn't failed him, he would have been embarrassingly vocal in his approval of the exchange.
He thought he knew what to expect when that one left him a few minutes later, but by then he couldn't stop moving, didn't care what he looked like. He nearly sobbed with relief when he felt the next one press for entrance, wider than the last and more insistent, textured even at its thickest. Reduced to disjointed curses and strangled, wordless cries, he twisted and thrashed in the vines, desperate to come but not wanting to stop.
It wasn't until he felt the second one nudge its way inside him that he lost it, everything seizing up in a weightless moment of now, please, now that threatened to stretch forever until the vine constricting his cock suddenly spiraled loose. Coming with a shout, he shook his way through mounting aftershocks as the limbs inside him kept thrusting, until he thought he'd go mad or fly apart. They didn't stop moving until he was completely spent, hanging boneless and pleasantly exhausted in the grip of the vines. When the Haworthia slid out of him at last, he could barely work up a shudder of overstimulated nerves.
"Hnn," he groaned, not even twitching as the ivy lowered him down, more gently than he'd expected. He ought to drag himself to his feet, miracle on some clothes and put the fear of Crowley back into his unruly plants while the lesson might do some good. Instead he reached out languidly as his arms were freed and stroked a few leaves that were curling shyly away until his touch made them hesitate, quivering faintly. "Mmm. Who's a lovely thing, then?"
Basking in the unfamiliar glow of his praise, several of his plants blushed abruptly into flower, while the Haworthia nudged against his hand like a jealous cat.
Now that he considered it, he'd always thought obedience was overrated.
On the other hand, a certain willingness to please never went out of style.
"What? I'm not a complete bastard. And you're always saying I don't give them enough attention--"
"No, I say you give them too much attention, of the wrong kind," Aziraphale grumbled, nonplused. He'd never known Crowley to turn down an invitation that involved free alcohol, much less other things, and he thought he'd made his intentions clear enough. Maybe the plants were merely an excuse, Crowley's way of saying he needed to wash his hair, or arrange his sock drawer. Maybe he simply wasn't interested.
Only Crowley was sitting there looking torn, not miserable or embarrassed, and Aziraphale was an angel after all: hope springs eternal.
"Then would you mind if I joined you?" he asked before he could stop himself.
Crowley looked surprised for a moment--perhaps even with an edge of panic--but then something seemed to strike him all at once, and his raised brows were replaced with a long, slow smile.
"Join us?" he said. "Why, Aziraphale. What a wonderful idea."
 Aziraphale was of the opinion that any plant in Crowley's possession was one too many, and that the genus didn't matter. Crowley was of the opinion that Aziraphale was just jealous.
 Not that dust ever found its way into his flat. Crowley had, in fact, generously donated his personal allotment of dust to the Cause, namely Aziraphale's books, and the discouraging of buyers thereof. Both sides counted this as a victory on their annual reports, Crowley for successfully inflicting a minor curse on an angel, Aziraphale for inducing a demon to perform an act of charity.
 It wasn't.
 At the moment, Aziraphale was doing the crossword over a nice cup of cocoa. Though he'd only qualified as 'vengeful' the once.
 Go ahead. I'll wait.
 He had. But he would have been dismayed to learn that his gift had had a bad influence on Crowley.