This is a hotly-debated topic online with hobby orchid growers assuming, 5.8 pH is the magic pH, where you get optimum nutrient availability and uptake in orchids; however, I call bluff on the necessity of that exact pH for all orchids. If you want to read more about pH for orchids, I’ve written this detailed post addressing pH for “normal orchids” (epiphytes) – the ones that are not covered in the edge-cases of this article you’re currently about to read.
First – You have many species of orchids that grow wild on limestone cliffs (some phals, vandas, phrags and paphs included) where the pH of rainwater is constantly being buffered up to create alkaline conditions at the root zone.
Second – You have species native to acidic niches and regions of South America (and likely other areas too), where the pH is extremely low, and the nutrient ratios are low in standard minerals, but high in things like iron.
Third – You have an issue and a question to address, “How universal are the studies done on pH and nutrient availability relative to all orchids?” Most of the available literature on the topic comes from the study of crop plants (cannabis, tomatoes, etc) and those findings are erroneously applied to orchids. Let’s get real here…”crop plants” are terrestrial and require very specific conditions (and ample nutrients) to survive at their fast growth rate. Also, like the greater world of non-orchids, there are many species that specifically require acidic or alkaline conditions.
We should be pragmatic and recognize orchids have adapted to survive where other “fast-growing plants” will not. Orchids have adapted to grow slower and steadily in volatile areas where non-orchids simply cannot survive. Orchids have a uniquely slower metabolism, slower growth rate, and slower nutrient absorption rate–this is why they’re able to grow in many places most common plants cannot and that’s why we should be more criticle of the conditions we provide them (and not let “easy plants” and their care muddy our understanding of an orchid’s requirements.
There ARE Orchids that do Better With a Higher pH
This isn’t a question–it’s fact. That’s why people add limestone to many orchids to improve growth.
Orchids that do better with higher pH are lithophytic and terrestrial species native to the Malaysian Limestone Region (Malaya, Sumatra, Java, Sulawesi and Borneo), as well as orchids from the mountain base of South America (Peru, Ecuador, etc), and any other orchids that grow in, on or around mountainous areas with exposed limestone, basalt or marble (often areas that are close to waterfalls and rivers). Areas where the water is buffered up by the presence of calcium carbonate in the rocks and subsequently in the water table as well.
Which Orchid Species Do Better with a Higher pH?
Orchids from the above-mentioned regions of course! And likely hybrids of those species.
Examples of High-pH Growing Orchid include:
- Phragmipediums (besseae, fischeri, kovachii, schlimii, caudatum, caricinum, boissierianum, longifolium, warscewiczianum (syn wallisii), and pearcei),
- Phalaenopsis (maculata, modesta, fimbriata, sumatrana, lowii, corningiana, pulcherrima, braceana, viridis, and cochlearis),
- Paphiopedilums (armeniacum, bellatulum, stonei, sanderianum, lowii, esquirole…and many other Chinese paph species),
- Rupicolous Laelias (bradei, briegeri, lucasiana, esalqueana, milleri, etc)
- Vanda suavis tricolor
- Ludisia discolor and other “jewel orchids” from the above mentioned regions.
There are likely many other species (and don’t forget to include hybrids of those species) that will prefer higher pH, but at this point I’m still researching and exploring those specific orchids from those regions (which are of course not exclusively epiphytes). I will append this document as I find other species that prefer high-pH.
Photos of Orchids Growing Wild (In Situ) in, on, or Down Stream of Limestone or Basalt Rock in “High-pH” Environments
There are orchids species that need VERY ACIDIC conditions
It shouldn’t be surprising, but just like the edge cases of high-pH growing orchids, there are low-pH-loving orchids also. This info seems lost in most modern info (and maybe that’s because it only applies to a small number of orchids such as Cattleya lawrenceana and Phragmipedium kloschereanum); however, those species exist and are native niche habitats like the “Blackwater” regions of the Amazon and require an exceptionally low pH to grow well (likely because an abundance of iron at these low pH levels).
In 1983 G. C. K. & E. Dunsterville published an article in the May-June issue of Orchid Digest titled, “Blackwaters, Acid Rain and Blackwater Orchids.” The focus was on the blackwater region in the highlands of southern Venezuela, where C. lawrenceana and Phrag kloschereanum are found.
The rivers in this region are very acid, with pH readings as low as 3.0 in some areas. In addition, several scientific research teams have gone into the region, which is several hundred miles from possible industrial pollution, and measured the pH of freshly collected rainfall samples. Inexplicably, the tested samples all fell into the range of pH 3.5-5.0. To put this in perspective, household vinegar has a pH of about 5, blueberries, one of the most acid loving plants, prefer to grow in soil with a pH of 4.5-5.0, and rainwater normally has a pH of slightly below neutral (5.6-6.2).
The Dunstervilles were puzzled by the difficulty they had maintaining epiphytic orchids from the blackwater areas at their Caracas home. They indicated that the orchids seldom lived for more than a few years. They stated that the Caracas water supply, which they use to water their plants, is on the alkaline side with a pH of 7.0 or greater and speculated that the plants might perform better if more acidic water could be used.
So what does all of this mean for the average grower?
For starters, by and large a pH between 6-8 should be generally okay and will keep orchids alive; however, if you want to dial in your care and provide optimal conditions or if you want to grow orchids that are commonly considered difficult to keep, then consider adjusting your pH. Your goal is to match the balance of nutrients your plant would have had access to in its native environment. pH doesn’t automatically make those nutrients available (you still need to add them). However, pH can alter which nutrients are more soluble and available to a plant, and different nutrients are available at a high pH vs. low pH. If you want to learn more about pH in relationship to orchids, read this post on pH, adjustment and orchids.