Phalaenopsis – terminal stem – myth or reality?

Probably all amateur orchid growers of the Phalaenopsis genus have faced the “nightmare” of terminal spikes so far. And, as in any online environment with opinions expressed, expert views are often outnumbered by those of people who argue only from their limited experience with a few cases encountered, or personal opinions, the community of orchid-growing internet users is no exception to this phenomenon.

Although in very rare cases it may mean the end of the life cycle, most Phalaenopsis specimens with terminal spikes deteriorate due to growing conditions. Some enthusiasts mistakenly consider them lost plants from the start, completely ignoring their potential.

To have a justified opinion, it is necessary to gradually formulate and address a few hypotheses.

1. What are terminal spikes?
2. What triggers their appearance?
3. What effects do they have on the plants?
4. What actions or measures should be taken?

What are terminal spikes?

A terminal flower stalk is represented by the emergence from the central vegetative growth area of a bud with subsequent floral development. More precisely, in the leaf crown area, for example, in species of the genus Phalaenopsis, instead of a new leaf, a flower stalk appears. This aspect is known in orchid species with monopodial growth, generally belonging to the tribe Vandeae, from the subfamily Epidendroideae, which includes the subtribes Aerangidinae, Angraecinae, Aeridinae, Agrostophyllinae, Bromheadiinae, Polystachyinae. It is important to note in this context an important aspect related to the biology of Vandeae, namely the type of monopodial growth, which involves a single growth direction, located at the apex (at the top of the plant), opposite the root zone, from which the roots emerge, in contrast to species with sympodial growth, where there are multiple growth directions. In the case of monopodial species, we can assume that there is an apical dominance probably controlled hormonally, which suppresses the development of lateral buds into shoots or implicitly separate plantlets. If instead of the usual leaves that indicate that the plant is in full growth, elongating its stem unidirectionally, a flower stalk appears, it is reasonable to assume that the respective plant will cease foliar growth in that direction. However, this aspect does not mean that the plant will die or enter a stagnant period, since the cancellation of apical dominance, in most cases, activates the development of lateral buds through plant hormones or growth regulators from the cytokinin group. Essentially, this phenomenon of lateral bud development is also observed in treatments with cytokinins, either in the form of keiki pastes, foliar treatments, or even unintentional destruction of the terminal bud. Moreover, it can be observed that the appearance of terminal stalks is more frequent in species with a higher capacity to produce keiki, and the ability to form new lateral shoots is frequently used in some Aeridinae, for example, Vanda, Mokara, Trichoglottis, by applying the technique known as topping or decapitation, where by removing the terminal bud, the appearance of adventive plants growing on the basal area left after decapitation is stimulated. It is interesting to note that the dormant buds of Phalaenopsis species have developmental versatility, being able to form vegetative buds, which will give rise to plants, namely keiki, or equally can form floral buds, which will lead to the emergence of flower stalks. This aspect is exploited by administering hormonal treatments with cytokinins, more precisely keiki pastes, which convert flower stalk buds into vegetative buds. In this sense, we can agree that this conversion is possible and occurs spontaneously, induced or naturally, also in the case of vegetative growth buds, which become floral if certain conditions are met. And from here, terminal stalks.

What triggers the appearance of terminal spikes?

The appearance of terminal spikes is caused by the conversion of the vegetative bud of monopodial orchids into a floral bud and can have multiple causes, physical (mechanical decapitation of specimens, intentional for reproductive purposes, or accidental), chemical (there is information that some antifungal, antibacterial, insecticide, or miticide treatments, applied abundantly, can cause this event), or biochemical–physiological (naturally hormone-induced – at the maturity age of plants or artificially, by administering cytokinins). Ultimately, all these causes induce the production or metabolism of cytokinins, their accumulation in tissues, and the activation of lateral buds, which will eventually lead to the production of keiki, new plants emerging on the mother plant's stem. Some species and varieties are more susceptible to forming terminal spikes, and it is interesting to note that the same varieties are also those where the appearance of basal plantlets (keiki) is more frequent. Some growers link the reason for the appearance of terminal spikes either to excessive light radiation or to the use of systemic insecticides, but there is no certain information about this phenomenon to date.

What effects do terminal spikes have on the health and development of plants?

Although floral spikes naturally appear in the Phalaenopsis genus in basal or lateral positions, it is possible for them to emerge in apical positions as well. Usually, this phenomenon triggers the appearance of basal plantlets, i.e., keiki, since the respective plant will no longer grow but will continue to function perfectly normally biologically, producing flowers and investing much more energy in vegetative reproduction.

In many cases, as informed by AOS experts (Questions About Phalaenopsis), the appearance of terminal spikes is an indicator that the respective plant has reached maturity and will stop upward growth, but there are numerous known cases where plants continued to grow after the floral spike dried out. A certain aspect in treating plants with terminal spikes is that the respective plant will certainly produce at least one basal keiki, depending on species, variety, hybrid, since it is known that some varieties are more prolific than others.