Drove over my flowerbed

I'd say childish !
Like a naughty little boy told not to do something, he straight away went and did it!
Large stones or rocks are quite a good idea although not sure that it would stop him doing it again.
I don't really understand why you can't point out to him that what he did was wrong - and pointless. Yes, you may rely on him to do certain things for you and he knows that and holds you to ransom by his behaviour, thinking he can get away with it because you won't say anything.
You either have to just accept that the wild flowers will grow again and hope he doesn't do it again and put up with his form of bullying or stand up to him at the risk of losing his help, but possibly being able to get that help from outside your family or maybe from another family member.
Age UK offer a lot of services that might be useful to you as back up .

Rocks idea very good and no more dinners for son

If deliberate, that’s abusive.
If accidental - well, let it go - the rocks idea is good!

Rocks. not saying where though

I'd slap the back of his legs

Would be normally need to drive in that direction?
If so, maybe he was forgetful.
If not, he was nasty.

Put some rocks there and draw them to his attention.

Actually quite savage if it really wasn't necessary and straight after you'd been talking to him about your wildflower patch. Is he nastily disrespectful generally?

I would be having words !! I would also explain how upsetting it was to see the lovely flowers mown down and would say I am very disappointed at his thoughtlessness.

What `Hetty58 said!

I'm with you Hetty
A few large rocks or similar.

I'd put a few large rocks there to stop it happening again!

Greenlady, I do not know. He is very good at handling a car. As timetogo says not nice either way.

He takes offence very readily, and is also a big help to me in important ways. He also says how much he appreciates the lovely dinners I make for him. The fact is he is unconscious of wild flowers, never sees them, which unless someone has paid money for them, he calls weeds.

Not nice either way.
I would make him park on the road in future.

accidentally or on purpose?