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How to keep landscape grasses from spreading
Confine the root system.
Cut the bottom out of a large plastic pot. Use heavy shears or a stout knife to cut the bottom out.

Available options: [+] If you will be fertilizing outdoors, there needs to be enough room to secure the roots on the bottom into the soil. Cut away a large piece of soil covering the roots, plus just enough to allow the shoots to waft the bottom out in front of you.; [+] Coat each cut area with a very thin layer of compost or manure. Discard any or all of the compost.; [+] Make sure not to cut too thinly. About 24 inches (61 cm) of hole pierce the bottom of the pot with the thickest cut for the root system.; [+] The pot should be 12 " (30. 5 cm) or more across and at least 8 " (20.;
Answer:
The pot should be 12 " (30. 5 cm) or more across and at least 8 " (20.