Titchmarsh, who came under fire in 2010 for working with B&Q and the BBC at the same time, is partnering with the DIY and garden centre chain for a second year.
"I'm happy everything I've suggested to improve what B&Q was doing was without exception taken up and improvements were made," he said. "That was my sole reason for agreeing to take on the job. The alternative was turning my back on the largest garden centre chain in Britain.
"I got the chance to make the plant material better quality and the staff that bit better informed on what they're selling. What we all want in gardening is for people to come back. There's no point in making a fast buck."
He added: "There are few garden centres that grow their own plants. They are all buying stock from the same places as B&Q, so wouldn't it be better if the large chains were better at it? I want gardeners to be successful. If plants get to the garden as quickly as possible they are more likely to grow well."
On potential conflict of interest issues when presenting the BBC's RHS Chelsea Flower Show coverage despite the presence of a B&Q garden, he said: "I don't have that problem because I'm not going to cover the B&Q garden."
When asked about TV gardeners Carol Klein and Monty Don advocating growing your own or using specialist nurseries to buy plants, Titchmarsh said: "There is room for both. It's unrealistic to expect every gardener in British towns and cities to be supplied by small specialist nurseries."