Work began to extend the centre in July and is expected to be completed at the end of February.
The garden centre, on Newark Road in South Hykeham, was first built in 1971 and originally covered 125 square metres.
The new extension will include a food hall, featuring an award-winning butcher, a wide range of gifts and a covered outdoor plant sales area.
The popular restaurant on site will also be extended, making it one of the largest garden centre restaurants in the country. The space will also be hireable for talks, demonstrations and events.
The number of free parking spaces available on site will be increased by approximately 100 bays, with additional disabled parking.
The main contractor for the extension is Maher Millard and the architect is Evolution Design.
Chairman Richard Pennell said: ?The development of the centre is very exciting for us and will provide an improved shopping experience for our customers with a wider range of products.
?However the heart of the business will continue to be our love of plants, gardens and gardeners!?