One of the aspects of orchid culture that has frustrated me over the years is the description of fertilizer applications for different species and genera. I wonder if the authors, many of whom are quite accomplished growers, realize that their recommendations actually aren’t very usable? The key piece of information is usually missing — what fertilizer are they are starting with?
What I am referring to is the commonplace recommendations of “dilute fertilizer” or something like “1/4 or 1/2 strength” fertilizer. Quarter or half strength of what? Did they start with, say, Dyna-Gro Orchid Pro at 7-8-6, an MSU formula of 13-3-15, or a Peters formula of 30-10-10? There’s a major difference between 7% nitrogen, 13% nitrogen, and 30% nitrogen, not to mention the additional effects of phosphorus and potassium.
What I would like to offer in this article is a straightforward description of my fertilizer program. I hope that it might help you think through your fertilizing routine or provide a place to start to see what works best for your growing conditions. I have developed my protocol through a lot of reading and reflection, with consideration for the biochemistry of water/media interactions and the natural context of nutrient availability and absorption for epiphytes.
For starters, I almost exclusively use reverse osmosis (RO) water. The “almost” applies only to outdoor Cymbidium and Australian Dendrobium in the summer. We installed a set of three commercial-grade water filters on our garden house for misting and additional watering (see photo). It is like a heavy duty Brita filter: removing chlorine, chloramines, and larger cations. When needed, I provide a second weekly watering using the garden hose to save time and RO water consumption.
Otherwise, it’s RO water for everyone, every day and every week.
RO water is what we humans drink and cook with in our home, and, for similar reasons, I would like to keep the orchids free from having to metabolize all of the “stuff” in our heavily treated (and awful-tasting) tap water. Our tap water was reading dissolved solids of 250-300ppm and pH of 9.0 last year, but dissolved solids have dropped recently again; there seems to be some mixing of water supplies going on, possibly to deal with drought shortages. I don’t need to introduce such unpredictability into something as core to my orchid culture as food and water.
As a note, there are plenty of excellent references on water quality and fertilizer selection if you do use tap water. I do not feel qualified to recommend something that I do not use or practice. If you would like to learn more about the more technical aspects of water, fertilizer, and media interactions, I highly recommend the five articles printed in the Journal of the International Phalaenopsis Alliance in 2003. Links can be found at the be found at the bottom of this fertilizer page on Repotme.com. (I’m providing the link to their page since they have already received reprinting permission from the author).
Nitrogen concentration across the genera
Ok, with the background out of the way, here’s what I do … and I’m very pleased with the results. If I make changes, I’ll update this article.
From my perspective, low concentrations of fertilizer applied frequently are more natural and useful to orchids than higher concentrations applied infrequently. In natural settings, nutrient concentrations are very low, and regularly washed down onto the plants with frequent rain. I’ve seen several references to the natural availability of 25ppm nitrogen in a rain event, but do not know how widely that has been tested or researched. But it is useful as a conceptual starting point for fertilizer application.
My indoor orchids, which are mostly Paphiopedilum and mounts/baskets of highland Dendrobium, are fertilized 2x/week at low concentrations (25-40ppm N). I would find it desirable to be able to fertilize my outdoor orchids this way as well, but, right now, without an automated watering system and the majority of my orchids potted in Orchiata bark, this is not practical, or desirable for the wetness of the media. These outdoor Cymbidium and Dendrobium are fertilized weekly.
With those generalizations, below are the concentrations that I use for my fertilizer routine, with a general cycle of 3 applications of fertilizer and 1 pure water only. As a note, I usually do NOT water first, then fertilize. The exception is during a heat wave, with higher temperatures causing the media to be drier than normal. Then I water first, to ensure thorough hydration of the media, roots, and plant. Often, in this circumstance, I water the day BEFORE I fertilize.
Orchids would naturally be exposed to nutrients only at the start of rainfall, with the “food” particles washing down from the trees as the rain “washes” off the surrounding vegetation (or rocks). As it continues to rain, they would be receiving more and more pure water. More effectively, I usually cycle the benches twice with the fertilizer-water, to ensure that the potting media fully absorbs the water and they have two thorough rounds of moisture.
Dendrobium: 50ppm N. I offer this year-round for some species, such as those from New Guinea, and mid-April to mid-September for Australian Dendrobium, and those with a winter dry (or drier) rest. For my mounted species, which are watered daily, I offer 25pmm N in two weekly feedings. For the outdoor potted Australian Dendrobium, there is one weekly feeding of 50ppm.
Vandaceous and Cymbidium: 50-100ppm N/week in the growing season and 25-50ppm N/week in the colder months. We also add some 6 month time-release granular fertilizer to some of the Cymbidium pots that prefer a bit higher nitrogen. In particular, Cymbidium madidum hybrids grow more vigorously than many of the smaller southeast Asian Cymbidium species. Depending upon temperature and humidity, I generally do not water the outdoor Cymbidium at all in December, and then every 2-3 weeks until it warms up again. All of the vandaceous orchids move inside for the winter, and adopt the 2x/week feeding schedule with the mounted orchids (that is, 25ppm N 1-2 times a week).
Overall, I am less concerned about fertilizing from December to February; sometimes there will only be one application a month, and only if the weather is warm and sunny. We have been experiencing increasingly variable seasons here in central California, with increasingly variable growth periods for our orchids. I determine when I water, and if I fertilize, by the timing of active growth and temperature, not the calendar.
Phalaenopsis, Paphiopedilum, and other genera: 50 ppm N/week. From November-February, I generally decrease the fertilizing to 25-50 ppm N/week. The Paphs and Phals are watered and fertilized twice a week (25ppm) for most of the year. In the winter, when it is cooler, I water once a week and fertilize once a week or less. I use a mix of medium Orchiata bark and New Zealand tree fern for my Paphs and Phals; it is both moisture and fertilizer retentive, similar to sphagnum moss. However, it is also more “airy” and better suited to drying out in our cool, damp winters. (I do not use sphagnum moss because it tends to stay wet for too long during our winter rainy season.)
What fertilizer do I use? I use the MSU formula (13-3-15-8Ca-2Mg) for RO water for the Dendrobium, Vanda, and Cymbidium. For Phalaenopsis and Paphiopedilum, I use the “K-lite” version of the MSU formula (12-1-1-10Ca-3Mg). More on that below.
I have not seen any compromise in root, leaf, or flower growth at the lower fertilizer concentrations. My Cymbidium produce five bloom stalks without any bloom boosters. The reason that I give a range of nitrogen concentration (e.g., 50-100ppm) is that I will vary the concentration depending upon the weather. If growing conditions are favorable, then more. If it is likely to be less than optimal (unusually hot or cloudy), then a bit less for the coming week.
One of the benefits of lower fertilizer concentration is less fertilizer accumulating in the potting media. I suspect that this accumulation accelerates the break down of potting media, and certainly contributes more toxicity with accumulating residues. My potting media generally seems “fresher” than that of most orchids that I purchase. I attribute this to both the use of RO water and the lower fertilizer accumulation over multiple years. I credit my husband for pointing this out, when he observed the difference in several cymbidiums we repotted. We recently acquired some cymbidiums for the first time from a grower that uses RO water exclusively for them. They were in their pots for several years and had pristine roots, even with some degradation of their media.
Fertilizer homogeneity and stock solutions
If compounded correctly, all sources of a given fertilizer concentration “should” be the same content, but I’ve heard from a lot of people that their experience calls this into question. I purchase mine in bulk granules and make a 12.5% liquid stock solution to mix weekly. This helps deal with the heterogeneity of the fertilizer granules. If you look closely, the granules are different sizes. If you take a teaspoon or so at a time, then you are far less likely to actually be dosing correctly and getting an even distribution of 13-3-15 (or whatever). However, if you mix 5 pounds of granules at a time, then your resulting stock solution will be MUCH closer. I purchase my granular fertilizer from Kelley’s Korner Orchid Supplies.
For more information on making stock solutions, and creating YOUR desired ppm from the fertilizer that you are using, check out the First Rays Orchids website. I couldn’t do a better job than Ray in the details, so please learn from the source!
As a reference, I choose to make a 12.5% stock solution, stored in 1 gallon heavy duty plastic jugs, which provides 100ppm N at 1.5 Tbsp of liquid fertilizer concentrate per gallon of water. This is a pound of granular fertilizer to 7 pints of water (1:8 ).
I find it convenient to use “100 ppm N” as a starting point to dilute to 50ppm and 25ppm as needed for different genera. For example, when watering the outdoor Cymbidium and Dendrobium, I mix the fertilizer at 100ppm N, and then dilute in half in the watering can for the Dendrobium.
“K-lite” formulas: low phosphorus AND low potassium
You will note above that I use two different fertilizers: the “regular” MSU and the “K-lite.” Over the years, despite a lot of attention to cultural details, I noticed a pattern of root loss in Paphiopedilum and Phalaenopsis after 2-3 years of growth. It seemed more than just natural root cycling with age and new growth. Also, many Phalaenopsis showed unexpected light sensitivity — purple leaves, even on white-flowered orchids. In the course of my contemplation, I found this article on “What do Orchids Eat?” by Rick Lockwood, discussing potassium toxicity in his collection.
Many of his observations resonated with my experiences, and I switched over to K-lite for the Phalaenposis and Paphiopedilum. Why not the others? Well, they were doing quite well and I saw no need to change what was working. Plus, at the time, maybe 10% of my collection is Phals and Paphs, so it was a small scale experiment. After six months, I definitely noticed the decline of purple leaves in the Phals. After a year, the root health of the Paphs was outstanding, and the growth and flowering have continued to be as good or better than before (even at lower fertilizer concentrations). Right now, I have several dozen blooming size and NBS Paphiopedilum, and about 250 Paphiopedilum seedlings, that are happy with their meals. I’m not going back to potassium.
Monthly Fertilizer Supplements
While I believe that the consistency of the above fertilizer application, and use of RO water, is the core of my feeding program, I also use these supplements to enhance plant nutrition.
From April to August, once a month, I add a kelp extract for root growth stimulation and micronutrients. I use KelpMax from First Rays Orchids, with excellent results.
Monthly, for the “no fertilizer” flush week, I add Quantum Orchid, a probiotic for orchids. The difference was notable when I began using it, and I can see the difference in newly acquired orchids when they begin receiving it. It is like probiotics for humans — you don’t notice changes in your health every time that you eat yoghurt. But if you stop receiving probiotics, or some stress causes a decline, then you recognize the ill-health in the absence. I consider my monthly dosage of Quantum Orchid to be good maintenance of the probiotics in their media.
Magnesium sulfate (Epsom salts)
From April to September, I add supplemental magnesium sulfate once a month to their water. Magnesium is critical for chlorophyll development. I use a hydroponic grade granule that dissolves readily at a concentration of 1/4 tsp per gallon. I usually add it with the Quantum Orchid above.
I also have also trialed providing a relatively “high” dose of magnesium sulfate (0.5- 1tps/gallon) in the autumn for the Cymbidium. A large part of our growing area receives natural 50% shading from a deciduous tree. There is a strong increase in light over a short period when the leaves begin to fall. When the leaf drop begins, I provide the extra magnesium. In about 10 days, the Cymbidium leaves are glowing vibrant green — it is a remarkable result. The ability to produce and repair chlorophyll from the sudden light increase is supported by the magnesium. Currently I give two applications in September and October.
The inspiration for this “high magnesium” application came from a Paphiopedilum forum. Someone wished to try to induce blooming in a multi-floral Paphiopedilum by exposing it to an extra high level of light. It was recommended to provide increased magnesium to counteract damage to the leaves. I do not know if it worked for that person or Paph, but our Cymbidium have benefited from this approach.
I add Pro-Tek silica on a “as-needed” basis, with fertilizer, to support the orchids during high heat conditions. This means once or twice a month during the summer months (June-August). Silica is an important component of cellular structure and not usually available in water or fertilizer. I have excellent results with all outdoor plants in their drought and heat tolerance. Pro-Tek raises the pH of the fertilizer mixture, so I do carefully check the solution to keep the water in the range of 6-6.5. Generally, I only need a bit of “pH down” to compensate — a teaspoon per gallon of apple cider vinegar (acetic acid) works fine. Alternatively, I add Super-Thrive, which is quite acidic.
Super-Thrive is a proprietary blend of B vitamins and growth stimulants. My husband has outstanding success with vegetable starts and seedlings with Super-Thrive, and I’ve regenerated declining houseplants to a remarkable degree with it. For orchids, we soak Cymbidium divisions in Super-Thrive water for 30 minutes or so prior to potting. I will also provide it to orchids that are stressed, either new plants, after repotting, or occasionally the entire collection during high heat. For the latter use, as described above, I combine it with Pro-Tek silica solution for balanced pH.
My best wishes to you in your orchid growing! I love to feed people and plants, and I believe that most recipients would agree that they become healthier and happier over time as a result. I hope that this detailed description will support you in designing your own orchid feeding program. Your plants will appreciate your time and effort.