I shall obtain this article and give it a good read. This citation is probably of the defra contract report:
Conducting the Life Cycle Assessment of Tomato production in England and Wales was a challenge. Many areas of horticulture are very heterogeneous and it is often hard to interpret the few national statistics in terms of definable representative production systems.
The full citation and abstract are below
•The production method used has great importance in defining the total level of GHG.
•The most emitting inputs in the nurseries are plastics and peat.
•“surface unit” was used as functional unit to compare different kind of nurseries.
•The plants grown in a nursery can be considered as carbon sinks.
•Other impact categories can be useful to evaluate the environmental impact of nurseries.