shovel prune
v. To dig up a garden plant and discard it.
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Hill and McBride adore roses — each count about 25 bushes in their yards. But Hill emphasizes they are not fanatics. "If they need to be shovel pruned [gardening lingo for given the heave-ho], I'll do it," she said. "I'm not going to be upset about losing a $12 rose bush when it's past its prime."
—Janis D. Froeloch, “Gardening Neighbors Keep Running For Roses,” The Tampa Tribune, April 18, 2002
1989 (earliest)
Now, when the earth is soft and the weather is cool, is a good time of the year to consider rejuvenating and enlightening that precious garden space. Shovel-prune those misfits and replant with trees or shrubbery that are more in scale with the size of the garden and house, creating a more pleasant and productive homescape.
—Joyce Smith, “Get rid of plants that don't belong in their garden spots,” The Orange County Register, March 04, 1989
This phrase is a play on the verb prune (from the mid-16th century) which means "to cut off superfluous branches from a plant to promote fuller growth."