South Mountain reservation is just one hour away from NYC by NJ Transit train. The eponymous mountain is a basalt ridge, and is part of the Watchung mountains aligned from north to south. Although it is located a densely populated area of NJ, there are not many people on the trails and it feels like remote wilderness.
The park is crossed by the Rahway river, and one point of attraction are the Hemlock waterfalls, where the river falls over vertical basalt walls.
I had been there many times, but now I ran the photos of the place through the iNaturalist species identification machine. It was surprising to learn that many of the plant species in the park are invasive species, not native to North America. The dominant species is the Japanese knotweed which grows vigorously along the wet areas, and the Japanese barberry which dominates the sunny spots. Indeed, there are still the rhododendrons and the occasional hemlock trees but they are not that widely spread.
It would be interesting to monitor the spread of the invasive species, and see how the native species are holding out. I will be taking iNaturalist along the way at the next visit.
Update (2019). In the fall of 2019 I spent almost every weekend with good weather hiking the Lenape trail in this reservation. This is marked with a yellow vertical band and runs north-south along the ridge of the mountain. More careful observation revealed that the native flora is still alive and well, although in places it is obscured by the invasives.
Starting in the parking lot near the Millburn train station one climbs among beech trees with a few tulip trees (with Batman-like leaves) near the water reservoir close to the top. With some luck one can find a tulip tree flower on the ground. Late in the fall there are many of them but they have already turned to seeds, still arranged in a cone-like flower.
At the top of the climb, along the asphalted road loop there are pin oaks and red oaks, but if one follows the yellow marked trail among the trees one finds a small group of hickories, near two benches close to the top of the mountain. There are shagbark and pignut hickories, full of nuts in the fall. Eventually they ripen and fall to the ground later to the delight of the chipmunks and squirrels. The ground is covered with their shells split into near thirds. Near the Washington Rock there are more shagbark hickories, one of which is shown in one of the observations.
Another interesting sight around Washington Rock is a small group of post oaks, with cross-shaped leaves, which I have not seen elsewhere in the park.
The trail continues downhill from that point, and passes along a few other interesting trees. More hickories, but also an American Hophornbeam, full of the characteristic looking flowers/seeds. Observation attached.
Deep in the woods there are many witch hazels which seem to come in two types: with more rounded and with pointed leaves. In the fall they have small nut shells which are full of surprises . After a few days of storage they explode suddenly with a loud bang (preferably at night, to maximize the surprise effect), and eject the seeds to the most unexpected places. The phenomenon was described by Thoreau and is as surprising now as it must have appeared to him. Quite a Walden Pond moment. Even later in the fall they bloom and produce the nice delicate green-yellow flowers unlike anything else (saffron perhaps?)
Closer to the waterfalls there is a group of sassafras trees on the right side of the trail. Worth trying chewing on a green twig which has a very pleasant flavor. Native Americans must have used them as chewing gum.
On the slope leading down to the waterfalls there are many rhododendrons which bloom earlier in the fall and display beautiful flowers. More rhododendrons on the Rahway trail (white marked) which can be used for the return to the southern end of the park.
Some illustrations of these plants are shown in the observations linked.
