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Here's a slideshow of the University of Hawaii's Lyon Arboretum, at the uphill end of Oahu's Manoa Valley, next door to Manoa Falls. It's at the same city bus stop as Manoa Falls, so it's just as easy to get to: Ride bus #5 to the edge of suburbia, get off and walk uphill, and follow the signs. Or, of course, you can just drive there if you have a car, which I didn't. Then go to the little visitor center next to the parking lot, drop a few bucks in the donation box, and get a map. You're going to need the map, because you'll probably get lost. I did, briefly, and I almost never get lost. Having a map at least gets you un-lost eventually. There's mobile phone service around the visitor center -- some of the plants in the adjacent garden even have QR codes to scan for more information -- but cell service quickly fades out once you're in the forest, sadly preventing me from going on an Instagram rampage while wandering around, or from checking Google Maps while I was lost.
I'd love to be able to tell you all about all the tropical plants here, or at least about the ones I have photos of. The place is kind of overwhelming, though. I spent a couple of hours here and felt like I'd barely scratched the surface. I skipped most of the various side trails and took the main trail to the far end of the arboretum, trying to find the waterfall. Which is a different, and much smaller (and less impressive) waterfall than Manoa Falls. This seems to confuse visitors a lot. I had a group of Japanese tourists ask me for directions to Manoa Falls, and they were a bit crestfallen to find out they were in completely the wrong place. I gave them directions and later ran into them on the Manoa Falls trail, and they thanked me for pointing them in the right direction. So I felt like I'd done my good deed for the day.
It would be really easy to spend an entire day here, taking it slow and just wandering around looking at things and filling up a memory card with flower photos. Though I'd recommend taking the Manoa Falls trail too, for contrast. If you only visit the Lyon Arboretum, you might come away thinking this is what a regular Hawaiian rainforest looks like, and not realize how much selective planting and pruning and manicuring has gone into it.
Most plants aren't labeled, so knowing your way around tropical plants would enhance the experience, I'd imagine. I was surprised to learn that a heliconia is not quite the same thing as a banana plant, if that gives you some idea of my inexperience with tropical plants. The arboretum specializes in heliconias, ginger plants, palms, and bromeliads, among other things, so it wouldn't hurt at least know what those look like.
Two items of practical advice. First, there are mosquitoes. Wear DEET, or cross your fingers and try some supposed DEET alternative, or wear long pants & sleeves and hope for the best, whichever option you prefer. Second, it rains a lot here. 165 inches per year, or nearly half an inch per day, on average. The arboretum is just a few miles up the road from Waikiki and downtown Honolulu, but it's not unusual to have torrential rain here while it's sunny at the beach. Oahu microclimates are like that. At least the rain isn't cold, and individual storms don't seem to last long, so you can sort of work around the weather and explore between downpours.