Hartley said: "Garden centres' sales progression is good but deceptive because they are selling more and more non-gardening products."
He added that the garden centre sector "is the slowest to respond to the internet and that's why it is under pressure from the internet" though, he added, it was slightly immune because of nature of product sold.
Gardening sales now account for less than 50 per cent of overall sales according to GCA figures Hartley showed.
He added that the conference venue Wyevale Melbicks garden centre, because of the amount of concessions and catering, was "no longer truly a garden centre, it's something of a misnomer".
Younger people are losing interest in DIY and are not buying their own homes so DIYs are rationalising stores, said Hartley, who added that 'getting someone in' was doing better than DIY post-recession.
The industry analyst said that the Screwfix trade outlet was doing better than B&Q, though both are owned by Kingfisher.
Housing starts are up from 100,000 in 2009 to potentially 225,000 in 2019 and 175,000 in 2015.
Hartley said building houses adds one per cent annual growth in housing stock and in Britain little was demolished -less than 10,000 a year - meaning more gardens were created every year.
The market is split catalogue/mail order/internet 15 per cent, builders merchant/direct 21 per cent, DIY 31 per cent, garden centre 23 per cent and High St/supermarket 10 per cent, he added.
GCA product split data showed catering was 17 per cent, horticulture 21, furniture 7, hard landscaping 3, sundries 15 and other 37.
Hartley said DIY product split was chemicals 14 per cent, sundries 25, horticulture 13, garden buildings 6, garden leisure 18 and garden equipment 24.
Garden online spend was now £400m+, up from £100m in 2008 and rising to £700m in 2018.
And finally, internet product mix was leisure 31 per cent, equipment 22, chemicals 2, sundries 13, horticulture 22 and buildings 10.