Using worsted weight yarn and 4.5 mm needle, cast on 11-14 sts.
Knit one row, purl one row, knit 1 row, purl 1 row, knit 1 row (5 rows of stocking stitch ending with a knit row).
Next row, *knit 2 tog, knit to last 2 sts, k2tog. Next row purl, next row knit (3 rows). Next row knit twice in first stitch, knit to end of row, increase one stitch in the last stitch. Next 4 rows: Purl 1 row, knit 1 row, purl 1 row, knit 1 row.*
These 8 rows make the pattern repeat, do this repeat 6 times and then knit one row, purl one row, knit one row. Cast off.
Sew cast on and cast off ends together. (The 5 rows of reverse stocking stitch are on the right side of the pumpkin, in other words the "bumps" of the 5 stocking stitch rows). Now using a length of your yarn and darning needle pass the needle through the bumps of the row ends on one end of your pumpkin. When you have gone all the way around draw the yarn tight (just like a draw string) to bring the top/bottom of the pumpking together. Stuff the pumpkin lightly with cotton, wool or whatever you have on hand and run the needle and yarn up through the center of the pumpkin and stuffing and gather the row ends of that end the same way. Draw them tight and also the yarn running from top to bottom of the pumpkin to give it some shape. Darn in this end when you like the shape you have.
Make a stem out of brown or green yarn. Cast on 4 sts on a double pointed needle and knit one row. Don't turn your work, just slide it back to the beginning of the needle and knit the 4 sts again. Repeat this until the stem is desired length and cast off. Sew this to the top of the pumpkin.
Variations. You can just knit a strip in garter stitch to what ever length you like, and sew it up the same way. The verical lines of the garter stitch make good pumpkin ridges. (Pumpkin on the left).
Use any weight yarn you have on hand and the appropriate needle size, you will get a larger or smaller pumpkin.
Embroider a Hallowe'en face on your pumpkin, knit a leaf for the top of the pumpkin, whatever you like. This is a chance to be creative. After all, no two pumpkins are alike and doesn't have to fit anyone!