Take a look at this graph. This graph shows the growth of one of our dragonfruit plants from August 31 - May 31. Measurements of our tallest plant was taken at last day of the month. At first it may spook you - it is statistics. What also might spook you is the sharp peak between April and May. Why is that? On the last day of April, the Dragonfruit was growing outside. It survived 5 months of winter and temperatures below 65ºF (which is extremely common here, and extremely uncommon in it's native ranges). On May 1, I put a plastic ziplock back on the container.
A gardening site says this about yellow (palora) dragonfruit:
Most of the days in SF between December and March do not exceed 65ºF. All 4 dragonfruit's survived these "extremely cold" temperatures. How could this be?
Take this analogy. We, humans, are one species. However, depending on the regular climate of the area you live in, you can be more cold/heat tolerant. For instance, if you are from Alaska and you move to San Francisco, you will think that SF is hot. Therefore, it will be harder to tolerate the city's temps. If you move from SF to Los Angeles, you will definitely feel a lot hotter than an LA native.
And if you take a Californian and move them to New York, you won't be as used to the extreme temperature swings and high humidity. One time has past, you will adapt to the surrounding climate and be able to thrive more happily.
Put this into the context of the fruit. The dragonfruit variety we have is the Ecuador Palora (Selenicereus megalanthus) and the fruit sticker said it was grown in Central America. In Central America, it is hot and humid, so when the seeds were planted in SF, they did not grow as fast. But, as the sprouts got use to the weather, and as the weather warmed up they started growing more.
As winter faded into spring, I decided that it was time for the Dragonfruit plants to experience some "real" spring. I brought them inside, put them in full sun, and then I was away for 2 weeks.
The picture above is on May 13, which was the day before I left the plants under a bag for 1 week.
Why does humidity make the plants grow faster? In contrast to most cacti, the dragonfruit species are extremely fond of humidity. The sun shines more along the equator than other parts of the world. This causes more evaporation, which causes more humidity. The water condenses into storm clouds, which explains the intense rainfalls the tropics occasionally receive .
Dragonfruits are use to the humidity, and long ago, the species broke away from the desert-dwelling cacti to form a new humid-tolerant group.
Another reason dragonfruit like humidity is because their liking of moisture. Moisture from monsoon season and water vapor are critical for dragonfruits because the pads are made of 95% water. Unlike Most cacti and succulents from the Mojave Desert, the Dragonfruit uses it's water to grow readily. In contrast, California cacti like the Saguaro do not need as much water because they have it stored in their flesh for when they really need it.
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