Welcome to the Halleck Park Arboretum! The cache is not at posted coordinates, but that is where you will begin the Letterbox directions. There are two ways to get to the final location: option one is the easier (and more enjoyable) method: simply follow the letterbox directions. Option 2 is a bit harder than the posted D rating: solve the puzzle at the bottom of this page.
Fair warning: you'll need a long TOTT at the final. Please make sure the container is completely sealed and returned to its correct location when you're done.
Option 1: a walk through the park
There are multiples of some of the tree species in the Arboretum, so skipping ahead is not advised. :) While most of the tour is on pavement, you will need to "off-road" a little.
You'll notice that there are two paved paths into the Arboretum from posted coords. Choose the path that has a Halka Honeylocust maybe about 25 feet to the right and start your journey. Take your first left and continue to the end of the pavement. The tree to your right is a Green Ash. Don't believe me? Read its sign. Then, with the Green Ash behind you, walk about 12 paces forward. In front of you, you'll see an Autumn Purple Ash. Turn right and continue down the paved trail. Walk past the young Yellowwood and follow the path past the Thornless Cockspur Hawthorn, followed quickly by a Canadian Red Cherry. Straight ahead you'll see another Green Ash. Go left on the path and you'll encounter a Northern Catalpa and shortly thereafter, yet another Canadian Red Cherry on the other side of the path. Follow the curve past a Kentucky Coffeetree and, on the other side, a Flowering Crabapple and another - you guessed it - Canadian Red Cherry. At the intersection, turn toward the young Eastern Redbud. Walk past the Skymaster English Oak and the White Swamp Oak. On the other side of the trail is a Pin Oak. OK, let's take a shortcut and do some off-roading! Pass the Pin Oak on your right and soon you'll come to a Black Maple. Pass the Black Hills Spruce and the Sugar Maple, then veer to the right and walk between the Red Spire Pear and the Amur Maple. The next tree to the right, whose initials are A.P.A., is where you'll find the cache.
Option 2: the puzzle
You can assume N41 0X.XXX W096 0X.XXX
Dragon Blood Tree
Norway Maple - Crimean Linden
Crimean Linden + Japanese Lilac + Russian Olive Tree
Monkey Puzzle
Paper Birch
Quaking Aspen
Three Kings Kaikomako - Quaking Aspen
Flowering Crabapple - Bur Oak
You can check your answers for this puzzle on GeoChecker.com.
Many thanks to Lori of WadeLori for helping me beta-test and revise my very first Letterbox!