A massive cumulonimbus rising high in the sky, driven by thermals rising from the hot dry ground. At the peak of the cloud, a beautiful cauliflower head is illuminated to bright white by the sun. The head of the cloud can keep on rising on vigorous thermals until it flattens when it hits the troposphere and makes the classic anvil shape.
In the most vigorous of systems these clouds generate tornadoes but all we'll get is a super thunderstorm, accompanied by heavy rain and hail. If we're lucky.
That'll make a mess of the begonias.