The Health and Safety Executive (HSE) prosecuted Westland Nurseries (Offenham) Ltd following the incident at the company's premises on 4 December 2009.
Worcester Magistrates' Court heard how the 21-year-old man, who does not want to be named, was cleaning a valley gutter between two roofs on a glasshouse when he fell about three metres through the glass, landing on a concrete floor.
He suffered concussion with blood on the brain, fractured his hand and sustained other cuts and bruises, and could not work for three months after the incident.
HSE's investigation into the incident found Westland Nurseries (Offenham) had taken no measures to prevent workers falling from the glasshouse roofs and did not understand the risks involved when working at height.
In addition, the injured man had been inadequately trained, by an employee who was not trained in working at height himself.
Westland Nurseries (Offenham) Ltd, of Ferry Lane, Offenham, Evesham, pleaded guilty today to breaching Section 2(1) of the Health and Safety at Work etc Act 1974 and was fined £12,000 and ordered to pay £6,835 costs.
Speaking after the hearing, HSE inspector Chris Gregory said:
"Every year, people are seriously injured as a result of falling through glasshouse roofs onto wires, hooks, staging or the ground, and from being cut by glass.
"However, the vast majority of these incidents can be avoided with proper planning.
"Companies must make sure they understand the risks involved in working at height and take suitable measures to prevent workers from falling, such as installing lightweight balancing frames.
"It is unacceptable that Westland Nurseries (Offenham) Ltd failed to do this and a man was seriously injured as a result."Guidance on working safely on glasshouse roofs is available from HSE at http://www.hse.gov.uk/pubns/ais12.pdf