I occasionally get questions regarding potting soil. I am posting an email reply that I recently made to a woman who was having difficulty with her new Hoya cuttings and plants. It may contain some useful information for others who are new to this plant.
Hi Rita, Do not despair! I have made many mistakes along the way including rotting all of the roots off from my plants when I tried a soil made of Turface and barkfines. I heard about this “miracle mix” on the Internet. It did not work because it was too heavy to be able to heft the pot to tell when to water. If I put my finger into the mix, the top 1″ was dry, but deeper was soaking wet. I quickly lost those plants. The moral of the story is don’t stray too far from the mix you know. You obviously have been growing plants for a long time, and know how to grow tropical plants. Hoyas are not that different from other plants.
Hoyas are very easy to kill when young, but not so easy to kill once they are established. That is why I keep so many large plants; they are much more forgiving of mistakes. It may take a while to find a mix that works for you; my mix works well for me; although I still occasionally have some problems when the mix does not get mixed well enough and a plant ends up mostly in peat. Wet and cool is going to rot your roots in quick order if your mix holds too much moisture. I think your instincts are correct, if your pots feel heavy to you, it is probably holding too much water. I have never used pumice, but it sounds a lot like turface, which was too heavy for me to tell when to water. It is very important to have an airy mixture that holds moisture for a few days. I think more than anything in my mix the large #4 perlite is the most important. Small perlite is no good. You need to have large chunks (1/2″) which hold moisture, but allow the mix to breathe. For the most part, I think your mix is fine, but I might try to eliminate the pumice and replace it with #4 perlite. Until those cuttings are well established, for best results they need to be warm, humid with an airy mix that hold moisture for a few days, but allow the soil to breathe. That is the best advice that I can give.
Don’t give up!
Doug