Height of Trees

[richg]

That is very easy to answer. Just divide any real size by 87 for the height you are interested in.
There are tricks in putting them in a layout though which has been answered.
Always remember, HO is 1/87 th of full size.
Keep a calculator handy and you will eventually be able to recognize sizes.

Rich

[Doneldon]

Cody-

I apologize for the goofy information and poor attempts at humor which some posters gave you when you were asking a simple, reasonable question.

Most of the information is good, though.  Keep the sizes within reason (4-6" max for HO), use different kinds of trees unless you are modeling an orchard, and put your best trees near the front of your pike.  As with other details, trees can be simpler as they recede from the front of the layout or are at greater elevations.  (That's a good way to force perpective a little.)  Many companies make trees but they can get quite pricey for a large number of trees.  You'll probably want more than you think before you actually start placing them.  One thing I do is check Christmas stores and departments for trees which are for the holiday villages, after the holidays when the stores don't want to inventory or keep them until next year.

There a a dozen ways to make trees.  Again, use the most elaborate for near trees with less complicated ones to the rear.  You can get kits for trees from various manufacturers including JTT and Woodland Scenics.  You can make trees using purchased armatures, dried weeds from your yard or the country, sisal rope or floral wire twisted together.  For foliage you can use ground foam, stretched ScotchBrite pads, lichen, or a dozen other materials.  Remember that all of the trees in nature are perfect.  That is, they are all real trees and perfect even though no two are identical or without a stray branch.  Consequently, make sure that your trees are perfect, too, that is, with imperfections and variations.  More clearly put, don't get compulsive when evaluating your output. 

Good luck!
                                                                                         -- D