The acquisition of Wyevale Landscapes and Wyevale Grounds Maintenance - the trading names of Western Landscapes Limited - takes the Landscape Group to more than £50m turnover for the first time.
A total of 140 staff will transfer to the Landscape Group, but the Wyevale companies are expected to keep their names for now.
Former Wyevale Landscapes managing director David Brew will become regional director of the Landscape Group for the south west.
Wyevale Grounds Maintenance, based near Bristol, provides hard and soft landscaping and grounds maintenance services to local authorities operating from 10 sites around the country including major contracts with the local authorities of the Isle of Wight, Lewes, Eastbourne, Stroud and North Somerset.
The firm has been trading in its current form since 1989 and before that as part of the wider Wyevale family of companies which have been trading since the 1950s.
The Landscape Group chief executive Nick Temple-Heald told HW the deal would allow for greater economies of scale.
"Critical mass is very important, and that will always help in terms of weathering the recession," he explained. "This deal really does add value to the Landscape Group."
The takeover took around five months to come to fruition, added Temple-Heald. It is part of an acquisition strategy planned since Temple-Heald joined the firm from Glendale last year.