Portas praised London centres Boma, Petersham Nurseries and Clifton Nurseries, while criticising Homebase and garden centres that stock resin birds, dead plants, cheap cuddly toys, plnats that have blown over and Haribo sweets.
But she said of Boma: "You have a lovely environment but I've never learnt what to do with my garden there. I've never been to a garden centre where anyone really educated me on what to do with my garden."
She told delegates:" It us vital to be able to accept change."
The Mary Queen of Shops star said she was seeing a change in retailing from "bling culture" because of economic crisis, environmental crisis and digital revolution.
She added that the environmental crisis is "a big plus" for garden centres, with four out of five in the US still buying green products despite premium prices and recession.
Portas added that because of the digital revolution consumers had "huge knowledge before they enter your garden centre".
She said touch, feel and service at shops has been lost through fast consumerism, adding that a new universal shopper is now likely to buy at Lidl and Waitrose.
Mary Portas did not name the garden centres she criticised but delegates felt they could have been any centre, although some said she should have looked further afield for more examples.
Portas said Clifton Nurseries' entrance felt "beautiful" and praised its "return to basics".
She said garden centres should offer additional services such as free garden design consultation and liked examples of colour blocking of plants and simple cafe food.
But she attacked bad merchandising standards such as pots that fall over, messy pondliner displays, lack of guidance on piles of compost bags and "don't give a monkeys" lack of inspiration. She showed a picture of a garden centre card/gift shop that "doesn't speak of specialism".
She said selling off dead plants harmed your business and asked of a yellow resin bird "who bought these?"
She added of a Haribo stand: "Why put it in a natural environment like a garden centre even if it brings you an extra £500 a square foot?"
On the subject of employees Portas said she only employs "happy people" because "you can get a heppy person to make a cup of coffee but you can't make a manic depressive happy".