Orchids, particularly cultivated Phalaenopsis varieties, are epiphytes, meaning they naturally grow on other plants, like trees, rather than rooting in the ground. This unique growth habit dictates that their roots are fundamentally different from those of terrestrial plants. Orchid roots should generally be exposed to air and light, as this exposure mimics their natural environment and is necessary for their specialized functions. Understanding this biology reveals why traditional gardening concepts of burying all roots do not apply. The root system is an adaptation that allows the plant to thrive where water and nutrients are only intermittently available.
The Biological Requirement for Air
Orchid roots possess a spongy, multi-layered outer tissue called the velamen radicum, which is composed of dead cells when mature. This specialized layer serves multiple functions, including the rapid absorption of moisture and nutrients from the air and rain. The velamen acts like a highly absorbent sheath, soaking up water vapor and rainfall before gradually releasing it to the living cells inside the root. This porous structure also allows for rapid gas exchange, which is necessary for the root’s respiration.
Unlike roots buried in dense soil, orchid roots require constant access to oxygen to prevent suffocation, a condition known as hypoxia. The velamen’s ability to quickly dry out—often draining completely in under two hours—prevents the roots from remaining saturated for long periods. This rapid wet-dry cycle is essential, as prolonged moisture blocks the necessary gas exchange, causing the root to rot. The need for air gaps around the roots is a requirement for the orchid’s survival, regardless of whether the root is inside or outside a pot.
Distinguishing Potted and Aerial Roots
The epiphytic nature of many orchids results in two types of roots: those contained within the growing medium and those that grow outside of it, known as aerial roots. Roots inside the pot are adapted to the potting medium, which must be chunky and open to simulate the air exposure they require. Media like bark chips, charcoal, or lava rock are used because they offer large air pockets, allowing for the quick draining and drying necessary to maintain root health.
Aerial roots grow freely into the open air and are adapted to drawing moisture directly from the atmosphere. These roots should not be forcefully buried into the potting mix during repotting, as the environment inside the pot is generally too moist for their open-air adaptation. Forcing them into a dense medium may cause them to suffocate and die. If aerial roots appear dry or shriveled, a light daily misting can supply them with the atmospheric moisture they seek.
Leaving healthy aerial roots exposed is the best practice, as they serve as a backup system for the plant to absorb moisture and anchor itself. These exposed roots are functional and indicate the plant’s overall health. While the roots are adaptable, a potted root will not transition easily to a dry, aerial environment, and an aerial root will often succumb to rot if placed into a perpetually moist environment.
The Role of Light Exposure
Many cultivated orchids, such as Phalaenopsis, have roots that exhibit a green hue when healthy and exposed to light, indicating the presence of chlorophyll. This suggests that the roots are capable of photosynthesis, a process typically associated only with leaves. While the leaves remain the plant’s primary site for energy production, the roots can contribute to the plant’s overall carbon metabolism.
Root photosynthesis may also serve a specific biological function unrelated to simply producing sugar. Research suggests that in the bulky roots of the Phalaenopsis orchid, oxygen diffusion can be limited, especially when the roots are wet or in the dark. The oxygen produced as a byproduct of root photosynthesis, when the root is exposed to light, helps to mitigate this internal oxygen deficiency, or hypoxia. Therefore, exposing these green roots to light is beneficial, not just for supplemental energy, but also to help ensure the roots can “breathe” from the inside. Completely white roots that have grown in the dark lack chlorophyll, which is normal and does not indicate poor health.