Marshall said he would "recognise and celebrate British horticulture which in recent years has suffered intense competition from abroad" at an event at Kew Gardens.
Post-Brexit, exchange rates have helped make UK growers' plants more attractive to buy, with Marshall saying that with the 15% difference plus transport, there were good reasons to buy British.
Some growers said if sterling strengthens that might end, but Dobbies new contract with Scotland-based Matthew Algie coffee, which it said was costing 30% more than Italian roasters, shows a commitment to UK sourcing that outweighs price.
Marshall said Dobbies was an "oasis in a world of cynical retail", toasting "great British nurseries".
Turnover has risen in recent months "above budget", while selling off what Marshall regards as inferior product means profits have not risen as fast. He plans a higher quality offer throughout the store, with new gifts, better food and new generation battery garden tools all coming in. He said "not knowing your customer has left us all reeling" in reference to the recent election.
Marshall said Midlothian Capital "fortunately" bought Dobbies from Tesco for £217m in 2016. He added that he "jumped at the chance" to become chief executive 14 weeks ago after investor Andrew Bracey asked him to join.
He said a "host of old friends" has joined the existing team. The aim is to be sector leader with plants centre stage with best quality, range and value for money and that would lead to "pushing at an open door".
Referring to an article in Horticulture Week, Marshall said: "Now is the time for British nurseries to step up."
He said linking with Ocado to up the "piffling" UK online plant sales percentage would tap into a market where "the sky's the limit - now is the time for all of us to step up to the plate". He said the £4bn UK horticulture market is ripe for online sales, as the rest of UK retail sells 40% online.
Growers suggested bedding, for instance, sells £25m online but could reach £100m. Some questioned how the difference between online plants and retail plants would be addressed. Dobbies' aim is to have a full plant and product offer online by spring 2018. Delivery could be direct from nurseries via Ocado, which has a 6m people database.
Growers such as Hawkesmill are now back at Dobbies, while many nurseries at the event were hoping for a share of the pie.
Growers present included Wyevale Nurseries, Allensmore, Lowaters, Wye, Kilworth, Golden Grove, Nursery Fresh, Hillier, Cobbins, Farplants, Bryants, Eastwoods, Garden Centre Plants, Bransfords, New Forest Plants, New Leaf, Quantil, Frank Matthews, John Richards, AE Roberts, Osberton, Pinetops, Hills, Whetman Pinks, Chapel Cottage, Chessum, Jersey Plants Direct, Harkness, Darby, Lovania, Porters, Taylors Bulbs, David Austin, Bowden, Blomfield and Hardy's.