Why do we speak, for example, of a 'common or garden' bicycle, meaning one that simply does the job of a bicycle without alloy wheels, Sir Bradley Wiggins pedals or any other bells and whistles.
'Common or garden' means much the same thing as 'bog standard' and can apply to anything animate or inanimate.
Edit 28/7/15
The OED entry says.
d. passing into adj., in the slang phr. common or garden, a jocular substitute for ‘common’, ‘ordinary’.
[1657 W. Coles Adam in Eden xxix. 59 But the Common or Garden Nightshade is not dangerous.]
1892 Autobiog. Eng. Gamekeeper (J. Wilkins) 67 It was as large as a common—or garden—hen.
1896 Daily News 16 Oct. 3/4 Such common or garden proceedings not being to the taste of Noa.
1897 Westm. Gaz. 4 Aug. 8/2, I have—to make use of a common or garden expression—been ‘rushed’ in this matter.
Best Answer
The derivation of the phrase obviously does have something to do with gardening, or more precisely, agriculture. Its original meaning, as has already been said, relates to the type of plant, fruit or vegetable which is found frequently in gardens or on "commons". (Historically, "commons" were the large patches of grass or woodland that ancient rural villages designated as being for the use of the community as a whole.) If such a plant is found growing in "the common or garden" it is likely to be unexceptional because of its abundance. The phrase has since come to be applied to anything that is common or unexceptional. (I was going to say "run of the mill" but that would be opening a whole new bag of worms, to coin a phrase).