Some garden centre and nursery staff may not have experienced the problems caused by the worst freeze for 20 years, say the HTA and retailers.
Containerised plants such as laurel, Magnolia, Camellia and Italian stock in garden centres are likely to have frozen roots.
Glendoick garden centre managing director Ken Cox said: "Because we've not had cold like this for years in garden centres all round the country they are going to have stock killed.
The Perth-based Garden Retail Award winner said: "You're not going to know until evergreen roots thaw.
"People who work in garden centre planterias may not have seen this before and it may be the same on nurseries.
"Camellias and magnolias leaves have crisped up so that suggests that's it for them. Also all the Italian stock sitting around the M25 will not be used to the root freeze.
Lots of garden centres are giving 5 years plant guarantees. I suggest we will be seeing signs being put up saying no returns."
Consultant John Adlam said: "This is a very difficult area because we're dealing with situations where in the past this has not been a big problem. There is the potential for root damage to the perimeter of pots."
Adlam said plants with lots of roots round the side of the pot would be damaged most. This could lead to plants being slow to establish in the spring and to them being susceptible to disease such as Phytophthora and Pythium.
He said: "The plant can manage if it goes into [freezing weather] slowly but there is the possibility because they went into this fairly quickly there could be more damage this year than previous years, especially to big pots and larger plants.
"The trouble with evergreens is they don't tell you when they've died.
"Garden centres will be legitimately selling some plants and not until April/May when they start browning off because people don't realise the roots are non-active. There is the potential garden centres will be selling plants that look healthy but won't be able to supply itself with sufficient water once growth starts in March/April/May.
Customers will take plants back to garden centres under plant return guarantees.
Adlam suggested garden centres use tetrazolium under manufacturers' instructions to check if frost has damaged roots.
But he said there was little legal basis for garden centres to return frost-damaged stock to suppliers and distributors. "With foreign growers and distributors there is even less guarantee. There is very little legal basis for this under the Sales of Goods Act."
Adlam said the Churchillian mantra of "jaw, jaw, not war, war" was the best way of sorting out problems: "Most garden centres would replace plants brought back but with the nursery that supplies them it has to be built on relationships and goodwill."
He said the UK could be zoned for plant hardiness to cold. He also said the term "hardy" needs better definition.
HTA business development director Tim Briercliffe said: "I expect damage to plants and garden centres need to keep an eye on that.
"If you sell a plant with a guarantee you have to replace it. If it died in the garden centre that would apply. But if dies in the frost in a garden they don't have to replace that plant-otherwise we would have no bedding industry.
"But it is very difficult to prove whether a plant died in a garden centre or a garden."
"These are exceptional conditions. If HTA member garden centres or nurseries aren't sure what to do they should ring John Adlam on the HTA nursery stock advice line on 01379 741999."
He added: "In last year's hard frost a lot of people lost Mediterranean plants in their gardens. It will be the same this year. But snow has an insulating effect too."
Adlam said it has been the hardest winter for 10-15 years, adding that many growers "have been lulled into a false sense of security on this".
He said controlled release fertiliser was providing too much nitrogen to plants in warm Octobers and Novembers, producing soft growth with little frost hardiness.
"These plants end up being damaged more by frost than those with lower nitrogen products."
He added that decades without very cold weather meant "lost techniques" such as gapping up pots to avoid cold damage were an issue.
Adlam suggested a return to the mid-1980s plant insurance schemes for crop damage.
He also said: "We are seeing more and more tunnel oproduction to meet stricter supply criteria. That is good from a frost point of view but if you are clearing heavy snow on multispan tunnel gutters read the code of practice.
Some HTA winter tips from the Nursery Stock Advice Line (email firstname.lastname@example.org)
- Water stress Knock out pots and check roots periodically. On sunny days when the foliage transpires, to avoid foliage scorch or even death of the crop, spray with an anti-transpirant like Wiltpruf S600 to conserve moisture or use white fleece
- Frost protection Covering plants at night with white fleece is superior to polythene for frost protection. Adlam recommends Cosytex.
- Salt damage During snow and frost, protect conifers from salt damage alongside the road by spraying with an anti-transpirant such as Wiltpruf S600.
- Root death test Conifers and other evergreens in particular take quite a long time to show up foliar symptoms of root death. A solution of tetrazolium will instantly tell you if a root is alive or not. Place a few roots in the solution and live roots turn red. Call the Nursery Stock Advice Line for an information sheet or details of supply.
- Split stems Some plants will suffer from split stems after a period of prolonged cold. This is not always evident straight away and you can despatch plants that may not survive. At the time of despatch preparation do check on rhododendrons and other ornamental crops.
- Mains water taps Any outside taps connected to the mains should be checked for leaks as soon as the temperature rises again. Leaks and bursts can create an expensive water bill.
- Tunnel heaters Open tunnel doors at least once a day even for a short period to change the air where gas or oil heaters have been running for prolonged periods. This prevents sulphur dioxide or ethylene foliar damage. Even if the heaters have a fresh air inlet it is worth it.
- Spraying Avoid all spraying during frosty conditions and this includes winter washes. Do not apply herbicide granules to frozen land.
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