Foliar Fertilizer

[brsucculents]

I have heard that foliar fertilizer didn't work on Orchids because there are few stomata and most are located on the underside of the leaves. But I got to thinking (which is always dangerous) that probably only applies to Catltleya. Would foliar fertilizer work on Catasetum or Stanhopea for that mater?

[estación seca]

This would be an excellent project for somebody with a microscope.

[AnonYMouse]

I recently read Ray's comment on this somewhere. Perhaps he will reiterate here!

Meanwhile I'll try to find where I read it.

---------- Post added at 06:36 PM ---------- Previous post was at 06:28 PM ----------

It was here: https://www.orchidsforum.com/threads...orchids.15592/

[Orchid Whisperer]

You may be able to get some fertilizer through the leaves, but the roots are better designed for plant mineral nutrient uptake.

[euplusia]

Following the advice of the botanical garden of Heidelberg I use Siapton for foliar feeding. It's a product for agriculture and contains amino acids and nitrogen as chelating agent. I use it in addition in the growing season.

[Ray]

Nothing is "all or nothing" in orchid growing, but is a matter of degree.

Some chemicals are readily absorbed through the leaves (mostly through plasmodesmata; stomata do not transport liquids), while others are not. That can be due to the size, shape, or electrical charge of the molecule or ion, and how well it interacts with the absorbing site.

Another factor is the leaf itself. Most terrestrial plants actively absorb through the foliage, but they also do all of their respiratory gas exchange through them, too. Orchids, on the other hand, as a way to be miserly with water, have shifted that exchange primarily to the roots, and in many cases, have developed thick, waxy cuticle layers on the leaves to further prevent water loss. Those layers can prevent the majority of liquids from coming into contact with absorbing tissues, all by themselves,

However, there are many orchids with less-developed cuticle layers, and it seems highly plausible that foliar feeding might be more effective with them.

I still would not make that my primary method of feeding though, as the molecular or ionic factors of the liquids still apply, and in nature, they plants see very low solute concentrations only at the beginning of the rains, then are washed clean, so I would be wary of damage by stronger concentrations of applied chemicals.

[AvantGardner]

I use high urea 20-20-20 in my spray bottle. I could see a darkening effect in my catasetums when I started this. On phals, the absorption is spotty, resulting in darker and lighter green patches on the leaves. Paphs darkened up a bit too, but nothing like ctsms. I figure if nothing else, when it rains the fert is washed over the roots (I grow outside in the summer).

[Fairorchids]

Unless you conduct a formal test on two groups (experiment & control), this is all opinions & impressions. Even if a plant grows seemingly better, it can be the result of the plant being a year older and better established.

For the test to have any validity, it should be done:
A. On multiple genera
B. On meristems with same starting size.
C. On at least a dozen plants of each clone selected, to minimize random variability.

[troy]

I know paphs and slipper orchids do have cuticles on the top of the leaves on the underside the stomatas open to transpire mostly during sunrise and sunset, my guess during that time they would be able to absorb water or fertilizer although it could interfere with transpiration, I would not recommend foliar spraying at dusk that causses crown rot

[naoki]

This is an old thread, but I came across a scientific literature, so I'm adding a bit of information.

Actually, stomata doesn't seem to be relevant for nutrient uptake from leaves from the results of the experiment below (and from the current understanding among physiologists). So it doesn't matter whether you spray the top of bottom of the leaves.

The leaves are less efficient than roots in absorbing nutrients, but it is not so bad. So foliar feeding does work in orchids.

Here is the paper I'm basing my judgement:

Susilo et al. 2013. The Uptake and Partitioning of Nitrogen in Phalaenopsis Sogo Yukidian ‘V3’ as Shown by 15N as a Tracer JASHS Journal of the American Society for the Horticultural Science 138(3): 229-237

They applied Johnson's fertilizer with isotope-labeled N to upper surface of leaves (UL), lower surface of the leaves (LL), young roots (YR), and old roots (OR) of Phalaenopsis. Johnson's has both NO3 and NH4 (mostly NO3), only NO3 was labeled. Then after 8 weeks, they measured how much labeled N got accumulated in the plant. Here is the results (Table 2 of the paper).

UL: 34.1 micro gram
LL: 25.2 micro gram
YR: 82.8 micro gram
OR: 60.3 micro gram.

So leaves are about 1/2 efficient. I wouldn't call it too bad, and I think foliar feeding works for orchids. So I apply fertilizer to both leaves and roots. I think crown rot due to water on leaves is mostly myth. If you look at the cell morphology, it isn't that easy for the pathogen to invade unless there is some injury. Similarly, our skin is covered by microbes, but we don't easily get infected as long as we are healthy.

Also, the upper and lower surfaces of the leaves don't differ in the uptake (the upper seems to be slightly more efficient, but the difference is not statistically significant). This is consistent with the idea that the stomata (more stomata in the lower surface) is not relevant for nutrient uptake. The actual mechanisms of foliar nutrient uptake isn't well known yet, but the plasmodesmata (tiny holes connecting cells to cells) on the surface cell (epidermis), called ectodesmata, seems to play a role.