Consultancy mdj2 associates director Andy Newman told Horticulture Week: "Christmas trees being sold from pub car parks is certainly not a new phenomenon, but it does seem more prolific this year."
He spoke out after garden centre chiefs told of unfair competition from tax-dodging "bit players" selling trees from garage forecourts for knock-down prices.
Newman said: "Any retailer should welcome competition, but only if it is a fair and even playing field, which in this case it is not.
"It is time for the garden-centre industry, led by the HTA, to start fighting back by launching a coordinated campaign to pressure local authorities and revenue & customs to enforce the law.
"Councils have an obligation to protect local business-rate-paying garden retailers from unlicensed traders - in most cases selling from a pub car park will be against that site's planning permission.
"HMRC also has an obligation to ensure traders are declaring their earnings, employing people legally, and paying the correct tax due - and we all know that is not the case here.
"Many of these pop-up sellers may be operating in the car parks of pubs and restaurants owned by major brewery or hospitality businesses.
"And this 'cash-economy' operation is certainly not going through their accounts either. So it is time for the garden centre industry to fight to protect their businesses."
Newman also urged garden retailers to keep pushing sales right up to the final closing hours of the nerve jangly week-before Christmas even though many had experienced sluggish sales to date.
"While the results so far may not be filling garden centre owners with festive joy, don't forget there is still a big weekend ahead, especially on trees.
"Christmas falling on a Sunday is a blessing as it creates a full shopping week beforehand. And when Christmas eve falls on a Saturday it is historically the biggest sales day of the year."
He said many of his friends and colleagues had still not bought a tree and garden centres had to offer the same level of customer service on trees this weekend as they had over the last two.
"Don't give up on them yet. Talking to directors of major high-street retailers this week they are all looking at figures that are down on last year so far.
"But they have high-hopes that next week can bring them back into like-for-like growth for the season. It will nevertheless be a very nervous few days ahead for retail."