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It’s Time for Food!

Folks spend a lot of time thinking about feeding their orchids. The most common questions are “What formula?” and “How much do I use?”. Both are valid questions, but one aspect that is often ignored is the role “time” plays in the overall feeding equation.

In fact, there are two important, interactive roles that time plays in a good feeding regimen: frequency and exposure. Let’s first look at them separately.

Frequency of feeding goes hand-in-hand with the formula and usage questions. I will use a simple analogy to demonstrate it – our own diets! Calories consumed are a major factor in a healthy diet. The same is true of plants, if you picture “amount of fertilizer” – more specifically, nitrogen – as being equivalent to “calories”, and “feeding frequency” as being “meals”.

An individual may do well with consuming 2000-2500 calories a day, and has the option of eating several, smaller meals or gorging on a single, large one. That same can be said for fertilizing our plants; we can choose to feed them frequently with a very dilute fertilizer solution, or we can feed less frequently, but with a “stronger” solution. My experience has taught me that orchids do quite well when they receive anywhere from 75 to 125 ppm N per week. By that I mean that if you sum the nitrogen concentrations of the solutions used over the course of a week, the total should be in the 75-125 ppm range. As examples, we have the option of applying a 25 ppm N solution 3 or 4 times a week, a 100 ppm N solution once a week, or a 200 ppm N solution every two weeks. Personally, I think that “more often/less concentrated” is better, as it more closely mimics what the plants receive in nature and have evolved to expect, and I recommend against feeding less often than every two weeks.

That leads us to the exposure aspect, which is rarely considered. First, some basics: roots absorb nutrients as ions in solution. When applied, the solutions are almost instantly absorbed by the spongey velamen layer on the roots, then slowly transferred through the roots’ cellular structure to the vascular system, which moves it throughout the plant. Let me use a few examples to demonstrate the “gradient” of exposure conditions:

  • For a vanda growing in a slat basket with no potting medium, the opportunity for absorption occurs only when it is being irrigated (or “fertigated”, since it’s irrigation with fertilizer solution), that is, the exposure is limited to solution directly contacting the roots, and the time is very short – when irrigation stops, nothing more can be absorbed.
  • Moving onto a cattleya in a pot of coarse bark, the exposure will be longer, as the bark will absorb some of the solution and pass it onto the roots in direct contact with it, even after active fertigation has stopped.
  • Next is a phalaenopsis in sphagnum moss. The contact area between the medium and the roots is far more extensive, and the moss will hold onto solutions a great deal longer than bark, so that “exposure window” can be many days.
  • Approaching the other end of the spectrum, for a plant grown in semi-hydroponic culture, with a constant supply of nutrient solution that is wicked throughout the pot, the roots are always in contact with the solution, so may absorb nutrients “at will”.

While the relationship between fertilizer concentration and feeding frequency is pretty straightforward and can be loosely “calculated” by summing the concentration of the solutions per week, it gets more complex when incorporating exposure time into the equation, but if you keep the two concepts in mind, you can “fine tune” your regimen to the needs of the plants.

Using our vanda-in-a-basket example from above, we might want to feed more frequently and with a little stronger solution, since the exposure time is so limited, while we’d be likely to feed less frequently and with a more dilute solution for a phalaenopsis in sphagnum.