"He had heard about talking to plants in the early seventies, on Radio Four, and thought it an..."

He had heard about talking to plants in the early seventies, on Radio Four, and thought it an excellent idea. Although talking is perhaps the wrong word for what Crowley did.
What he did was put the fear of God into them.

More precisely, the fear of Crowley.

In addition to which, every couple of month Crowley would pick out a plant that was growing too slowly, or succumbing to leaf-wilt or browning, or just didn’t look quite as good as the others, and he would carry it around to all the other plants. ‘Say goodbye to your friend,’ he’d say to them. ‘He just couldn’t cut it …’

Then he would leave the flat with the offending plant, and return an hour or so later with a large, empty flower pot, which he would leave somewhere conspicuously around the flat.

The plants were the most luxurious, verdant, and beautiful in London. Also the most terrified.