Gore Orchid Conservatory - Paphiopedilum
Fertilizing Slipper Orchids (and others).
Few things strike fear into the hearts of orchid hobbyists as quickly as the topic of fertilizer. It seems, to some, a bit too much like chemistry class... the shelves at the local nursery are piled high with bottles and jars and bags of fertilizers and suppliments, some for blooming, some for growth, some without urea, some with extra iron, calcium, magnesium .... the list could go on for pages.

I would like to be able to tell you that fertilzing is actually quite simple, and there's an easy method for determining the right way to do it, and you can treat all of your orchids the same way. In fact, I ALMOST can. The majority of orchids (especially hybrids) will do just fine with the same basic fertilizing program, but there are always exceptions, aren't there?

With this in mind, I'm going to start of with a few fertilizer basics. Just the bare minimum for the average person (who happens to grow orchids). From there, however, I'll move on to a practical discussion of some specifics for those who are growing tempermental orchids. Since I like species, I'm in this category.

I'll finish up with some data tables from my own experiments with various fertilzers and suppliments, which may be of use to those of you who start to get serious about fertilizer.

The Basics

Let's be honest here; Orchids grow slowly. At least, compared to many other plants, they hardly grow at all. Other plants may grow from seed to large flowering plants in a single summer; whereas it's not uncommon for some Paph species to take 7 years from seed to bloom. This is not meant to be discouraging. This single fact, though, is very important in understanding orchids and their needs.

Why do we fertilize plants? Simply stated, to give them the nutrients they need to grow. So, what if they don't grow much? They need very little fertilizer, of course. (Most orchids naturally grow in nutrient-poor substrates with diffuse light, and grow slowly to cope)

If, on the other hand, you refuse to accept that your orchid is slow growing and try to speed it up with fertilizer, what will happen to it? Well, first of all, it won't grow any faster (probably), and there's a very good chance that the plant will be burned by the excess of chemicals. The roots will be burned and will die, and the leaf-tips will start to turn brown from the concentration of salts.

This brings us to the first rule of slipper orchid fertilization:

1. Fertilize lightly. As a general rule, you should fertilize at half the strength suggested by the bottle.

IF IT IS SUNNY and your plant is growing, fertilize normally. If it's winter and you're getting shorter days, if your orchid is going through a dormant period, if its cloudy and the plant has been in the shade, fertilize LESS. Fertilizer is wasted if it can't be used for growth, and it may damage the plant.

and to go with it...

2. Flush with clean water thoroughly at least once a month. If you fertilize lightly but don't do this, the fertilizer salts will build up in your medium and burn your roots, just as if you had over-fertilized. I flush my orchids with pure water once a week if it's dry enough, and it usually is here in Colorado. If not, I skip a week in fertilizing and simply give a VERY heavy watering with pure water.

Ahh, but you're still wondering what fertilizer to use. To that end, let me relate a quick anecdote. Last summer (2005), I was visiting with Bob Ellis of B&B Orchids in Port Angeles, WA. Bob is a fountain of knowledge, a former nuclear physicist, and avid Paph and Phrag breeder- an intelligent man by any measure. He was showing me his recently FCC/AOS awarded Phrag. Jason Fischer. I had recently been experimenting with various fertilizers and suppliments, and I was so impressed with his plants (and his Jason Fischer in particular) that I asked him what fertilizer he was using. He pointed to a Tub of greenish fertilizer on a shelf near the door of his greenhouse; is was a simple, balanced 20-20-20 fertilizer from a local store (Peters, I think). Not formulated for orchids. Not a bloom formula. Not urea free. Not growth formula. Not expensive.

"Most professional growers use something like that," he said. "Don't bother with those specialty 'orchid' fertilizers - the plants don't know the difference." [

All that just to say that a simple, well balanced fertilizer (20-20-20 for example) will work prefectly for most of your orchids. If you're going to buy a special "orchid" fertilizer, look for one that contains Calcium and Magnesium as well. [I would be remiss if I did not also mention that Bob Ellis does sometimes add a vitamin B suppliment, which is intended to strengthen root growth].

In case you're wondering, the three numbers in a fertilizer title refer to three elements (macro-nutrients) that it contains: Nitrogen (N), Phosphorous (P) and Potassium (K).

Nitrogen is in high demand when leaf growth occurs, so high nitrogen fertilizers (e.g. 30-10-10) are usually called 'growth-formulas'. Phrags tend to grow faster than Paphs, and are frequently more leafy, but they grow slowly enough that extra N is not really required.

Phosphorus is important in general growth, but particularly in fruit and flower production. Again, a balanced fertilizer provides an adequate (in fact, excessive) amount of this element, so special blooms formulas with high P levels are not necessary, unless you are trying to force blooms. Many fertilizers balanced for orchids contain low amounts of P because higher levels do not seem to help, but they do make the fertilizer more acidic, and increase the total salts present. Michigan State University studies show that a ratio of somewhere around 12-6-13 provides ample P for orchid flowering.

Potassium is also important for many areas of plant growth, but most people focus on its importance for strong roots. It generally makes cells more sturdy, and can help increase the strngth of stems and leaves, and according to Dorothy Morgan, can improve cold resistance. Again, this element is ample in any balanced fertilizer.

Just Beyond the Basics

If you want to get serious about fertilizing, you'll really need a couple of special instruments. These are:

1. A pH meter.

2. A TDS (total dissolved solids) meter.

The pH meter is the more important of the two. It's the one to get if you can only get one. The importance of this instrument rests on the fact that orchids can only take up nutrients from their medium if it is in the proper pH range. In general, this range is between 6 and 7 for acidic growing orchids, and between 7 and 8 for lithophytes, but there are exceptions. Species specific pH information will be added to the culture pages some time in the future.

Fertilizer is typically acidic. When added to pure water, standard fertilizers can quickly drop the water pH to below 6. If your growing medium is already acidic (from decaying organic matter, especially containing peat or spaghnum moss), this can inhibit nutrient uptake in the roots.

If you're using tap water, however, this may not be a problem. In many parts of the country, especially those using well water, mineral content is already high enough that the water pH is substantially alkaline. If this is the case, the mix of the fertilizer and the naturally occuring minerals may react to balance the pH to an optimal level, between 6 and 7. The only way to know, however is to test it. That's where the pH meter comes in.

Even if this is the case, and the pH is optimal in the tap water mix, there may be another problem. Many Phrags and Disas are very sensitive to mineral content in water. If the level is too high, the plants grow poorly. In this case, it is important to check the TDS of the fertilizer mix. For Disas, over 100 PPM is high, and over 150 PPM is dangerous. Phrag hybrids can handle a bit more, but species fare best with equally low levels.

To the right, I have listed some pH and TDS measurements taken from common orchid fertilizers when mixed with PURE, RO/DI water. As you can see, all of the fertilizers are acidic, as is the suppliment "SUPERthrive". On the other hand, Pro-TeKt is basic, and can be used to adjust your pH back up.

All TDS readings are +/- 10 PPM, All pH reading are +/- .10
All readings are taken from the addative in one gallon of RO/DI water.
Dry fertilizer is not packed into measuring instrument. Powder is first stirred to a loose consistency. Excess is scraped from the top with a flat edged instrument.

pH readings are taken with a Hanna Instruments "Checker" digital pH meter.
TDS measurements are taken with a Hanna Instruments "TDS1" digital TDS meter.
TDS measurements are given in 3 numerals, as per the TDS1 read-out.


Beginning TDS = 001 PPM


1/4 tsp = 8 PPM
pH = 5.14

1/2 tsp = 15 PPM
pH = 4.98


Beginning TDS = 000 PPM

GROW MORE 20-20-20 (Yellow, "Premium Orchid Food")
1/2 tsp = 168 PPM
pH = 5.56

1 tsp = 317 PPM
pH = 5.39


GROW MORE Urea Free 20-10-20 (Green, "Premium Orchid Food")

Beginning TDS = 001

1/4 tsp = 146 PPM
pH = 5.77

1/2 tsp = 305 PPM
pH = 5.69

1 tsp = 565 PPM
pH = 5.64

Gore MORE Growth Formula 30-10-10 (Pink, "Premium Orchid Food")

Beginning TDS = 001

1/4 tsp = 046 PPM
pH = 6.15

1/2 tsp = 084 PPM
pH = 5.97

1 tsp = 154 PPM
pH = 5.84


DYNA-GROW Pro-TeKt 0-0-3 (The Silicon Solution: Nutritional Suppliment for Plants)

Beginning TDS = 001

1/4 tsp = 016 PPM
pH = 8.48

1/2 tsp = 031 PPM
pH = 8.54


ORCHIDMIX (pure Water Formula) 12-6-13

Begining TDS = 000

1/4 tsp = 141
pH = 5.79

1/2 tsp = 312
pH = 5.61

1/2 tsp + 1/4 tsp Pro-TeKt
pH = 6.71