The Dual Personality Of Orchid Roots

We often hear that orchids are difficult to grow, but this is only half the truth. Many orchids are actually quite easy to keep healthy. However, it’s important to understand the significant differences involved in growing them. In this column, we’ll focus on the particularities of orchid roots, which have a very different structure and function from conventional terrestrial plants.

The Orchid Root Is Coated With “Sponge”

As we explained in a previous article, orchids have a long history of adaptation to all kinds of habitats and radical climatic upheavals. Some orchids had to take refuge in higher ground to escape inhospitable soils, or to find the sunlight essential to their growth at a time when plant density was such that there was no longer enough light on the ground.

To survive their new life as epiphytes (i.e. clinging to trees), orchids have developed a coating around their roots. This spongy coating (known as velamen) is actually an accumulation of plant cells (more or less dead, by the way) that acts both as a sunscreen and an absorbent structure to catch rainwater. Some researchers use the term “root epidermis” to refer to the velamen, in analogy to our epidermis, which protects our internal tissues. This analogy is apt, as the velamen effectively protects the fragile roots from drying out – while allowing the gas exchange essential to the roots’ internal chemistry.

Depending on sun exposure (and other factors), the structure and thickness of velamen can vary significantly. The more intense the sunlight, the thicker the layer of protective cells. Conversely, when the root is developing away from the light (e.g. in a growing medium), the velamen is thinner and therefore less absorbent and more permeable to gas exchange.

The Aerial Root – Visible, Plump And Absorbent

The easiest form to observe is the aerial root (see photo CHR03_a). This is a root that develops in the open air and captures the maximum amount of rainwater during the slightest downpour.

If you cut an aerial root (photo CHR03_b), you can clearly see the vessels in the center, surrounded by the protective velamen (of a much greener color).

To better absorb the water stored in the velamen, the root produces a multitude of absorbent cilia (sometimes called “rootlets” or “absorbent hairs”), which are very difficult to observe, but are essential for water osmosis. These rootlets are very fragile and extremely sensitive to light. When the root is exposed to air, they remain well hidden within the velamen.

The anchor root – discreet, ingenious and very solid

In order to cling firmly to trees, epiphytic plants must also have anchoring roots that “weld” them to their host and enable them to withstand wind and other weather conditions. This second form of root differs from the aerial form in several ways.

The velamen, better protected from the sun, will be thinner, allowing the fragile rootlets to venture out of the velamen in search of moisture stored in the substrate. An example of this new conformation can be seen in photo below, where rootlets emerge massively from the immature velamen at the tip of a root. Other nearby roots have managed to maintain several absorbing rootlets, albeit more or less continuously. 

Observation Of Rootlets

Observing rootlets is usually not so easy because these structures are often ultra-thin and ultra-fragile. To photograph and study the fine structure of rootlets and their anchoring systems, German university researchers had to use electron microscopy on samples cooled to -140°C.

A researcher’s life isn’t always easy!
a) “molding” the velamen between the asperities;
b) wrapping the rootlets around the surrounding micro-asperities;
c) exuding sugars to “glue” the rootlets to the substrate.

The end result is remarkably strong, allowing the plant to remain firmly « planted » to its host, even when exposed to high winds and tropical storms. 

The Dual Personality Of Orchid Roots

n short, the epiphytic orchid will produce two slightly different types of roots: one aerial and optimized for collecting rainwater, the other for anchoring the plant firmly to its host. 

Although both root forms are composed of the same elements, their final configuration differs significantly, particularly in the length of the absorbing rootlets:

– short rootlets within the velamen in the case of the aerial root, or

– rootlets often elongated in the surrounding substrate to anchor and extract moisture and nutrients;

However, direct observation of morphological differences is very difficult due to the microscopic size of some absorbing rootlets, but also due to their great fragility when manipulated for observation. Anchor roots are often well hidden beneath the visible part of the root, making them very difficult to study.

Practical advice for growing orchids

Whether the root is in the air or buried in the substrate, it must perform the same two basic functions: water absorption and gas exchange. Gaz exchange should not be neglected  since it is mandatory to chemical transformation functions at the root level. In short, the root must “drink and breathe” just like us! 

However, these two functions compete with each other because when a velamen is waterlogged, the central root has difficulty breathing. It’s a bit like trying to breathe through a water-soaked washcloth pressed against your face. Air can’t get through (or barely gets through). 

So, for most orchids, we will alternate periods of hydration with periods of dryness to allow the root chemistry to be fully completed. 

But the hydration/respiration cycle will be very different depending on whether the root is aerial or in a substrate. For a typical epiphytic orchid root, the hydration/respiration cycle can be completed in 24 to 48 hours in the open air, but for a root surrounded by substrate, the cycle will take several days, often 7 to 10 days under normal growing conditions.

Watering

Orchids with only aerial roots (e.g. vandas or plate-mounted epiphytes) should be watered very often, as they will dry out much faster than their cousins in the substrate.

If the orchid has a good root network in the substrate, you’ll need to water less often, but enough to saturate the roots and the substrate with moisture. However, it is important to allow the plant to dry out between waterings to allow the gas exchange necessary for the internal chemistry of the roots. Dry periods also reduce the risk of bacterial and fungal infections. To ensure that the substrate is sufficiently dry before watering, you can simply lift the pot, which will become much lighter once the substrate is dry. 

Pour s’assurer que le substrat est suffisamment sec avant d’arroser, on peut simplement soulever le pot, qui s’allégera beaucoup lorsque le substrat sera sec. 

For the less experienced, I recommend the use of small bamboo sticks inserted into the substrate. These should be removed before watering to ensure that the center of the pot is sufficiently dry. Also, the smell of the stick will indicate the presence of bacteria or fungus in your pots long before you notice any damage to the surface. It’s easy, accurate and economical.

Thanks

The author would like to highlight the impressive research work done at the ‘Carl von Ossietzky’ and ‘Kiel’ universities in Germany. This work on vascular epiphytism is highly appreciated by orchid growers around the world.

Publisher’s note

Mr. Charpentier’s orchid chronicles are published on the site of the LaidBackGardener under a legal “Copyleft” format. As a result, republication of his columns is permitted provided that the following credit is added: 

This article was originally published in french on the Jardinierparesseux.com website and in the English on Laidbackgardener.blog.