The centre is close to the £5m Klondyke flagship Houghton Hall centre, which opened just three years ago and would lose 38% of trade if Dobbies opened, Dobbies' own research has found. Carlisle council said this was "not a material consideration" and was the "result of standard competition".
Klondyke has protested against the move but believes its centre alongside the M6 has a better site.
Plans for the 6,000sq m building and 408 parking spaces divided opinion locally, with 13 letters of support and nine against.
Dalston Parish Council said the centre is too large, would increase traffic levels unacceptably and result in a loss of trade to businesses.
But local businesses Orton Grange Farm and Dalston Aggregates said more shoppers in the area would be good for them.
The council hired retail consultancy DTZ, which said there was not enough "quantitative need" for Dobbies, but there was a "moderate qualitative need", which outweighed it.
Dobbies, which has 25 branches nationwide, submitted revised plans in October, responding to concerns about the building by lowering the height from 6m to 4.8m, and including a more discreet entrance and traffic island on the A595.
A report, to be considered by councillors on 13 November, concludes that "the issues are finely balanced" but the qualitative need for the centre outweighs the quantitative. "There would be no great harm in retail planning terms from permitting the garden centre," it says, and recommends planners approve the scheme.
Westwood Nursery, where the centre would be built, will supply the group with shrubs and bedding plants.
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