He keeps to the usual varieties, the types of chrysanthemums anyone can buy from a garden centre, and has never got into hybridising.
"I've never really been interested in cross-pollinating varieties and all that that involves," said Mr Robertson, who turns 84 in a couple of weeks.
"There are no real secrets, as long as you keep them well staked and water them when they need it. And a bit of liquid fertiliser helps."
But he said it was important to keep the blooms covered when the weather gets nasty during late season because heavy rain especially can damage them.
He used to grow up to 100 varieties under cover but now has cut that back to about 30.
"It's been a very funny season this year, either too hot or too cold, so it hasn't been easy getting the blooms at their best," Mr Robertson said.