She praised high-end London centres Boma, Petersham Nurseries and Clifton Nurseries, but criticised centres that stock resin birds, cheap cuddly toys, plants that have blown over and Haribo sweets.
The TV retail marketing consultant told garden centre owners that she had "never been to a centre where anyone really educated me on what to do with my garden".
The Mary Queen of Shops star said there was a change in retailing from "bling culture" because of the economic crash, environmental crisis and digital revolution.
She added that the environmental crisis was "a big plus" for garden centres, with four out of five in the USA still buying green products. The digital revolution meant consumers had "huge knowledge before they enter your centre".
Portas said touch, feel and service at shops had been lost through fast consumerism, adding that a new universal shopper was now likely to buy at Lidl and Waitrose.
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 pond-liner displays, lack of guidance on piles of compost bags and "don't give a monkey's" lack of inspiration.
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?"
Clifton Nurseries managing director Tad Paluchowski said: "Our focus is always on customer service, the highest-quality plants and accessories and a passion for what we do."
Palmers Garden Centre outdoor retail manager Ian Colledge said: "I learned an awful lot from her comments, particularly about the fact that we, as 'experts', don't always push to educate the customer enough."
HTA director-general David Gwyther said: "Mary's emphasis on the great opportunity for garden retailers with the 'new consumer' was heartening."