Many folks will tell you that vitamin/rooting hormone additives like SuperThrive or Dyna-Gro K-L-N are a boon to orchid growing, while others will tell you they are a waste of money. Certainly the apparent hype of some of the related advertising doesn't give one a lot of confidence in their claims! From a scientific perspective however, they should offer some benefit.
Based upon their labels, SuperThrive contains the synthetic rooting hormone naphthalene acetic acid (NAA) at 0.04%, vitamin B1 (also known to stimulate root growth) at 0.09%, plus a variety of trade-secret ingredients likely including kelp extracts, humates, soluble iron compounds, and the like. Similarly, K-L-N contains 0.025% vitamin B1 and 0.10% NAA, but also has another synthetic rooting hormone, indole acetic acid, or IBA at 0.05%. The hormones are very powerful chemicals, and with the B1, are known to stimulate the initiation of root growth. We all know the importance of a good root system, so why the mixed reviews?
One issue might be that of chemical stability. The hormones degrade quite rapidly upon exposure to heat and/or sunlight. A fresh batch of the concentrate, if kept dark and refrigerated, will be good for about a year. At room temperature, that drops to a matter of months, any any exposure to sunlight cuts its shelf life even further. This means, therefore, that purchasing old stock or keeping a large bottle around for a long time can ruin its effectiveness, so it is important to buy fresh stock and store it properly. Even so, do they really work?
I used SuperThrive consistently early in my orchid-growing, but was
taken aback by its price tag (ignoring the detail that the cost per
application - one or two drops - was insignificant). In early 1995,
after much research on potential ingredients, I made up my own improved
version from readily-available products.
After several months of adding one teaspoon of SuperThrive per gallon of fertilizer solution - 76 drops - things seemed to be going well, but when the phalaenopsis bloomed, the flowers were deformed. Fortunately, that was reversible, and upon reduction in dosing rate, the blossoms were fine in subsequent blooming cycles. I have heard anecdotal evidence that overdoing of the hormones can stunt vegetative growth as well, but have nothing to back that up.
In the years that followed I, like others, noted mixed benefits, and have come to the conclusion that if your overall culture is good, the hormones are of little benefit. If on the other hand, there is something "lacking" in your culture - old medium, inadequate or irregular watering, etc. - that causes your plants to become somewhat degraded, they can certainly help them recover more quickly. I suspect the large plants in my study, being "trapped" in plug flats, were under some stress. At this point, I still use the supplements, but only at repotting time, to help "kick start" the plants' efforts at getting established in the new pot and medium.