You can stitch its end closed and angle its base somewhat to make it sit at something other than a right angle to the trunk. 45 degrees is a good angle to aim for, but since mother nature has endless variations of angles available, there is no need to obsess over the exact angle.
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Add "spikes". Tie the yarn lengths every other loop along the ridges formed by your single crochet.
Trim the yarn ends to a length that looks pleasing to you (one quarter inch to one half inch, or .6 to 1.2 cm works well).
Glue the finished cactus into the pot if desired. Hot glue works well, as will craft glue.
Sew the branch to the side of the trunk using matching green yarn or thread.
Sew the trunk to the dirt using whichever yarn color will be least noticeable.

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Stuff your clay pot with a bit of batting.

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Place the cactus (and its attached "dirt" into the pot.

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Cut yarn "spikes" to length by wrapping it around something sturdy (three fingers work well) and cutting the resulting loops in half to create 1.5 to 2 inch (3.8 to 5 cm) lengths.
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