"They don't like direct sunlight, it burns the leaves," Theo says.
His collection is now past its peak for the year, which means there's a lot of work to be done in the coming weeks. Each plant, which has been hand pollinated, has its head cut off after flowering has finished. Watering is then held off over the following three weeks.
"In the fourth week the leaf has turned yellow so has stopped growing. They are then cut off and left in their pots, with no water 'til May," Theo says.
The tubers are then tipped out, washed and dipped in watered-down Janola.
"This kills any bugs in the tuber. Then I rewash them in warm water, dry them and put them in paper bags. They are labelled and stored for the winter."
While a quiet period follows, when spring arrives he is right back into it, sterilising pots and repotting the plants into both new and old soil.
"Water rationing is also a problem. I water them every third day by hand. I'm an early riser so have all day."
He says watering them by hand gives him the chance to check them out, where he might come across any bugs.
Theo says begonia hybrids were originally found in Bolivia in the 1800s, with no blue plants ever being grown.
"I tried for over 50 years. They are also mostly unscented, but I've managed to get a scented variety in the last five years."
Theo has given plenty of seedlings away over the years and now prefers growing begonias from propagation.
"I've gone back to leaf cuttings in pumice. Stem and basal cuttings give better tubers. They are too expensive to import these days."
Theo belongs to the Auckland and Canterbury begonia societies but no longer goes to shows. He says the Auckland group came down for a visit where he showed them the first plant he had developed and registered, which he named Greenlight.
"A judge from England also came and took a couple of tubers back. Everyone who saw it wanted one."
As well as growing his potted plants, he also grows the tropical variety which originated in Korea and are grown outdoors. He says a lot of the councils are now giving up growing them.
"They are too labour intensive so cost a lot to grow."
Along with the obvious pleasure Theo has gained over the years from his begonias, he also enjoys other people's reactions.
"They stand there and stare with their mouths open."
Although Theo may still be offered new plants from fellow begonia lovers, Betty has managed to discourage him from adding to his already vast collection.
"I can't handle any more, I've got to be sensible."