I must admit my oleanders which are now about 5 years old,do stay put,the are big and in heavy concrete pots,mostly south facing right next to the bungalow.They are actually coming out in flower now, everything being about 6 weeks late
You could get a bag of suitably sized pebbles from a GC (or stones from the garden) and use 3 per pot as 'pot feet', pushing them into the gravel to level each pot (with 3 points of contact, they will be stable.) Leave around 2cm of clearance under the pot so that any roots growing through are 'air pruned.'
A trowel in the hand is worth a thousand lost under a bush.
I agree with Bob, I would also raise them off the ground on improvised pot feet, sans saucer, on the gravel, just to be on the safe side. I would be concerned about even upside-down saucers that might trap rainwater inside the pot, unless they were somewhere sheltered from the rain.
Oleanders grow wild on riverbanks over here, roots in the water, including in a southern valley near my old house where it regularly got to -5 in winter and one year -12. They not only survived, but were a invasive menace there. Winters are short though, so that probably makes all the difference. In my current location in the north, they can’t survive our winter and early spring freezes without severe dieback, even on pot feet 😃
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
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Oleanders grow wild on riverbanks over here, roots in the water, including in a southern valley near my old house where it regularly got to -5 in winter and one year -12. They not only survived, but were a invasive menace there. Winters are short though, so that probably makes all the difference. In my current location in the north, they can’t survive our winter and early spring freezes without severe dieback, even on pot feet 😃