Yokohama Beach

Yokohama beach is on the Northwest shore of O'ahu. It is fairly popular among both locals and tourists alike and the climate here is much drier than at the other beaches. Even Ka'ena from the North side wasn't as dry as the approach from the West side. This is due to the West side being "shielded" by the volcanoes from the trade winds that help create condensation on the windward side of the islands. Condensation builds up as the trade winds blow clouds up and around the volcanoes on each island. However, the trade wind inversion, the altitude where trade winds stop rising over the volcano, prevents the clouds from reaching up over the whole island, so the windward side gets the majority of the rainfall. This is well-illustrated through mean annual rainfall collected in the Hawaii Rainfall Atlas. 

I began my trek on the West side near the Lower Makua Cave. I hope to get a chance to go exploring in there at some point in the future!

On the coast opposite the cave, the landscape is very rocky with some shrubby/succulent plants. I will go into describing these to the best of my abilities.

Grasses are very common here. Depending on whether there are people watering it or not, the grasses  are either lush and green or dried out and yellow. Here I closed in on some Fountain Grass (Pennisetum setaceum) that appears almost completely dried out but has strands at the top that resemble wheat. Among them are small 'Ala'ala wai nui (Plectranthus parviflorus) shrubs. These plants are in the mint family but have lost their minty aroma, which serves as a defense in nature, possibly due to lack of predators.

More fountain grass that hasn't completely dried out yet.

I spotted this plant among more grass, which I believe could be an 'Ala'ala wai nui that is flowering. However, the leaves make me think it is something else entirely that I do not see in my guides.

This little guy here is an Pigweed/Purslane (Portulaca oleracea). These plants are invasive to Hawai'i. It can be used in medicines, but it displaces native species.

 I believe this is an Alena plant (Boerhavia repens). It is a native plant that is used medicinally. The flowers are supposedly pink but I didn't see any on the plants that I saw.

The closest ID I could find for this tree was the Noni (Morinda citrifolia). This is based primarily on the flowers that are pictured on the left side of the plant. The plant has a number of uses in Hawai'i ranging from dyes to medicines.

 An Aloe plant (Aloe vera) growing among some non-native grass.

These trees were common at this location. Another is pictured above by the cave entrance. They are called Kiawe or Mesquite (Prosopis pallida). These trees generally have thorns on them when they are young. It was introduced in the early 1900s as a commercial plant with many uses, but it heavily threatens native plants by shading them out and absorbing any available water quickly.

I found this plant growing between lava rock. I was not able to confidently ID this plant. It looks like it could be several species in my guides and I don't know enough to call it one way or another.

A Spanish Needle (Bidens pilosa) plant.

 The next four images are of Koa Haole (Leucaena leucocephala). These plants were all over the place once I made it to Yokohama Beach proper. That and grasses, which were primarily the fountain grasses that were pictured above.

 Here we have some Naupaka (Scaevola sericea) that is found everywhere on the island.

A Pohuehue (Ipomoea pes-caprae) that I found on the beach itself. These plants are native to Hawai'i.

Summary:
Rating: 2/5
I didn't see much diversity in this location on my brief survey. A large amount of the flora here is comprised of either grasses, koa haole, or the kiawe trees. The dryness of the site itself also makes for a poor beach experience as you get blasted by the sun. If you hike up to Kaena from this side of the island, you can see tide pools along the way that people like to cool off in. This spot is also popular for surfers and fishermen, so this beach could be worth your while if you are coming for one of those or similar activities. However, as a recreational beach I wouldn't recommend this beach first, especially if you aren't already on this part of the island.

Citation:

Giambelluca, T.W., Q. Chen, A.G. Frazier, J.P. Price, Y.-L. Chen, P.-S. Chu, J.K. Eischeid, and D.M. Delparte, 2013: Online Rainfall Atlas of Hawai‘i. Bull. Amer. Meteor. Soc. 94, 313-316, doi: 10.1175/BAMS-D-11-00228.1.