Uncertainty among embattered nurseries - despite improved returns by growers in 2015 - is leading to a lack of plants in the market when sales boom and potentially leaving the door open to more imports.
HTA Nursery Business Improvement Scheme chairman Will George said hardy nursery stock growers have "no idea" how 2016 will work out despite a good year in 2015 so far.
Former Prenplants owner George added: "It has been quite a good year but members are worried it might just be the weather so they're not that confident for the future. People are cautious. They have no idea what's going to happen in 2016.
"There are major changes in the industry and garden centres are really cutting back on plants. We've had a good spring but there are no autumn sales now. Most garden centres are trying to clear out autumn. Most garden centres, especially the big chains, will have nothing by spring, which is good in a way because they fill up in March."
He said the market into garden centres is "very competitive, especially with so many large chains". Specialist plant suppliers such as the National Trust, English Heritage and private gardens now have more choice, while retail nursery internet sales are growing for the same reason. But mail order "is pretty stretched" because of the collapse of courier City Link and some couriers do not like handling big tree parcels that are bigger than 1.2m.
Liner producers are having to produce on spec because growers and retailers are not committing ahead, he added.
Mark Taylor, business development manager at liner producer Kernock Park Plants, agreed garden centre consolidation and a lack of certainty on forward orders are producing nervousness. "Anecdotally growers are just focusing on trying not to throw stuff away." he said.
Taylor added that the landscape of the industry has changed because many UK growers are producing less. In addition, garden centre buyers are concentrating on spring and looking overseas to top up because growers say they cannot afford to risk growing more in case of another year like 2013, when poor weather led to many retailers not taking reserves.
"There's a lack of confidence in growing more than a reserve," he confirmed. "We haven't overproduced. We've just produced what we think we're going to sell."
He added that with Wyevale and Blue Diamond expanding it is very difficult for nurseries to know how much to grow for next year. "Armitage's went to Wyevale last week - that's the sort of thing that makes growers nervous. They don't know who is going to own the garden centre for next season."
Garden centre plant sales were stopping by the end of June after a "one-hit whoosh" of stocking up in March, said Taylor.
Bransford Webbs Plant Company managing director Geoff Caesar said he is in the midst of a busy year but is "never going to be a huge amount up" on 2014 because the Hereford-based nursery is not growing much more than last year.
HTA horticulture head Raoul Curtis-Machin said growers are "always going to have an unpredictable market" and the HTA is focusing on Government procurement projects to give amenity growers certainty. But he said there is "no one-size-fits-all solution" for retail suppliers.
Garden Centre Association chief executive Iain Wylie said talking about autumn plant de-stocking is a "self-fulfilling prophecy". While plants are dropping as an overall sales proportion, they are still the "most important differentiation category", he added.
On retail consolidation, Wylie accepted that there is a "knock-on effect" that means growers are likely to have to consolidate too.
Business statistics - Self-sufficiency and import/export figures
The Farm Business Survey 2013-14 of horticulture production in England, from Rural Business Research and the University of Reading, has found that the UK was 49 per cent self-sufficient in ornamentals in terms of value. Vegetables were 38 per cent and fruit 16 per cent.
UK non-edible production has risen five per cent since 2006 but has changed little over the past three years. Imports in 2013 were up two per cent at £1,079m.
Cut flowers - roses (£168m) and chrysanthemums (£109m) in particular - pot plants (£127m) and bulbs (£85m) were the biggest imported crops, while exports were £65m, a rise of £13m on 2012.
There was a 12 per cent increase in the value of vegetable imports between 2012 and 2013, with a minimal change in the value of exports.
Fruit imports were up eight per cent at £2,956m, while exports were £111.5m, up £29m on 2012, including re-exports.
Meanwhile, latest figures from Defra show that plant and flower imports are rising while exports are falling.
The Agriculture in the United Kingdom plants and flowers annual update showed exports fell to £56m in 2014 from £61m in 2013. Imports increased to £1,106m in 2014 from £1,079m in 2013 (see p11 for more).
The latest UK/Dutch trade figures from LEI - the Netherlands national agricultural economics research institute - based on Eurostat figures have revealed that Dutch exports to the UK are worth a total of EUR1,108m, while the UK's exports to the Netherlands are valued at EUR191m.