Let’s talk a little about humidity needs for Hoyas grown by northern gardeners. I find it to be one of the most important Hoya needs, and probably the toughest one to provide for in the typical centrally heated home during the winter. I add between 6-10 gallons of water a day into the air in my home, and still when it is zero degrees outside, it is almost impossible to get the relative humidity above 35% in the main house. It does not take long for the effects of the dry air to be seen in most (but not all) of your plants. All of the newest tenderest growth will dry up and crisp, leaves will shrivel and fall off. When you remember how beautiful your plant looked in the summer, it is a little hard to take when try as you may, there is no new growth to coax out of your poor desiccated plant.
The easiest solution to this frustrating problem is to grow only those Hoyas that do well and tolerate lower humidity well. Here are a few that take dry air in stride for me: All of the carnosas, obovata, cumingiana, motoskei, pubicalyx, cv. Iris Marie, nummularoides, and kerrii,. The next solution is to add a humidifier which can keep your RH consistently above 40%. Then you can expand the group of Hoyas you keep, adding such species as lobbii, macgillivrayi, memoria, lacunosa, kenejiana, magnifica, javanica, diversifolia, polyneura, revoluta, imperialis, and bella. If you add grow tents, or a grow-room and can keep your humidity consistently above 60% you can add some of the following group: patella, elliptica, megalaster, kentiana, lambii, campanulata, and heuschkeliana. This is far from any kind of complete list, but are just examples of the way these particular plants behave for me.
Now the good news: Most of those dried out plants will begin to recover for you in the spring/summer as the RH continues to increase. It is a frustration we northerners must bear for falling in love with a jungle dwelling, humidity-loving, tropical plant!