The Valuations Office Agency (VOA) class these as nursery grounds, and as such require land to be used in conjunction with the growing buildings in order to satisfy the agricultural exemption.
This follows a Court of Appeal decision in 2015 involving a mushroom grower which led to a review of ratings.
Growers of young plants may have to pay business rates on glasshouses or buildings they grow in. The plants would be sold or leave the premises before they reach final maturity and there is little or no connected use of open land for the growing cycle.
Growers of plugs for sale to other growers are likely to be affected, as might bedding growers whose plant aren’t fully mature, even if they are selling to garden centres.
The NFU says the new "extreme approach" by the VOA "is contradictory to current guidance". Growers have told Horticulture Week they are "concerned".
Growers were covered by the agricultural exemption in the 1988 Local Government Finance Act.
Exemptions were for agricultural buildings "occupied together with agricultural land and be used solely in connection with agricultural operations on that or other agricultural land; or form part of a market garden and be used solely in connection with agricultural operations at the market garden.
For horticultural purposes, agricultural land is defined as ‘anything’ which consists of a market garden, nursery ground, orchard or allotment. But the mushroom case concluded that ‘anything’ does not mean buildings or glasshouses, but could mean growing frames, raised beds or trays.
The NFU says a nursery growing entirely under cover may not be exempt from business rates.
Letters could go to nurseries from VOA from 6 November, backdated to 2015.
The NFU wants to hear from growers and to start a campaign against the change.
Check www.voa.gov.uk to check liability.