Honestly, pumpkins will not do well at all in clay.
Go ahead and till a few small areas and build some 2x2 foot 6" high raised beds out of pressure treated, will cost about $7 for each bed. Space each of these beds 2-3 foot apart and fill each with 2 bags of "Miracle-Gro 1.5-cu ft Raised Bed Soil" for $9 each.
If you don't want to spend a lot, then only build 1 or 2 beds for now and add. Also consider growing them in 15 gallon grow bags which will require a little less soil (1-1/2 bags instead of 2 bags per container).
|