Heres the way i usually do papier maché trees for plays and puppet shows

1 : first cut out two times in card board a basic shape of the cactus you want ( each must be of the same height but not exactly the same design )
2 : cut out a wedge in the midlle of one of them from the bottom to the middle of the cactus
3 : do the same thing to the other one but the reverse  , from top to middle
4 : slip the card board together by the wedge and strenghten it by adding a cardboard slips of about 1 inch by 3 inches at random spaces all allong the edge and glue them with a hot glue gun
5 : glue a card board X to the bottom of the tree to add support or simply glue or nail it to a wooden board
6 : cut out cardboard slips and bend them to make the basic rounded shape of the cactus and glue them from side to side from the top to the bottom ( loops ) to make the other shape of the armature you just made.
7 : paper mache it with strips of paper : )


if the cardboard "sags" on its own because the piece is to tall , just glue more layers together..double "canelure"( sorry can only remember the word in french ) cardboard is the best to use..to know exactly how many "canelure's" your cardboard has just look at the side of your cardboard cut.. the "WWWWW"you see is one layer of "canelure" , the more layers it has the stronger the cardboard can support..it is VERY important to cross the weave of the cardboard when you mount the armature otherwise the will be no support in itself if one side should sag from overweight : )
Using a hard sponge afterwards to paint the finish adds a nice realistic pitted design by the way.

AHAHa that wasent my intention you know : ) i aint a big fan of chicken wire