not sure on how to make the tree but you can use Wilton tip #233 or #234 for the grass
The 'grass' tip is #233. There is a much bigger one to cover large areas but I'm not sure if it's 234.
As for making trees - one way is to use an ice cream cone upside down. Of course you want the 'sugar/waffle cone'. It can be covered w/tip 16 OR 65 thru 70 OR 233 
Start at the bottom and work up overlapping each row as you move up.
are you making the tree out of royal icing or fondant?
You could use a wafer cone (Not a waffle cone like for pine cones)
Stick a skewer through the bottom of the cone and into the cake. Fill the cone with cake scraps and/or frosting to the top so it has a rounded tree shape. Pipe the stem of the cone with brown icing for the trunk and the outside of the top with a small leaf tip. Does that make sense?
thank you . sure it makes sense . actually when kakeladi said ice cream cone the first thing it came to my mind was the other type of cone that looks more to a pine if used upside down but now that that you have mentioned the other type of cone, sure ,that it is the one i want to use which of course will look more to a regular tree. thank you for the idea how to to make the top part too because i never thought of that. thank you all of you who have answered and helped.
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