The firm, which operates from three sites in the Southport area, produces 25 million plants each year for clients including B&Q, Woolworths and local authorities.
Porters will expand its greenhouse site at Southport Old Road in Formby from 1.6ha to 2.4ha.
Director John Porter said: "The additional space created by the extension will be a real bonus for us. Not only does it give us the chance to extend the range of plants that we grow, it also means that we can make the growing process more efficient and reduce the time between planting and dispatch."
Funding comes from the Royal Bank of Scotland (RBS) and its asset finance arm Lombard.
RBS director of commercial banking Jeff Cummins said: "Porters is a good example of a local business that consistently performs well due to the quality of its products. The expansion will strengthen the firm's position in the market and hopefully provide it with more opportunities to develop the business."
The family-owned firm, formed in 1985, produces 150 different products including pot plants, hanging baskets, fuchsias, pansies and geraniums.
Porters' other locations include a 3.2ha site in Burscough and another site in Formby. The firm employs 30 people and has a turnover of £3.3m.