Plant Hormones
Plant hormones — sometimes called phytohormones — are chemical messengers that regulate nearly every aspect of plant growth and development, from a seed deciding it's time to sprout to a tomato deciding it's time to turn red. Unlike animal hormones, which travel through a bloodstream, plant hormones move through the plant in more varied ways: some diffuse cell to cell in a specific direction, others hitch a ride in the water and sugar transport system, and some are even gases that move through the air spaces inside plant tissue. Understanding what each hormone does — and how growers can work with them rather than against them — is one of the more genuinely useful things you can learn about how plants actually work.
A Bit of History: How We Discovered Plant Hormones
The first plant hormone discovered was auxin, and the story behind it is a nice piece of scientific history. In the 1880s, Charles Darwin and his son Francis ran experiments on canary grass seedlings, testing how they bent toward light. They found that covering the very tip of a young shoot with an opaque cap stopped it from bending toward light, while covering the base had no effect — meaning something happening at the tip was directing growth further down the stem. Decades later, researchers including Frits Went built on this work, using agar blocks to physically isolate and transfer whatever substance was diffusing out of a cut shoot tip, proving there really was a mobile chemical signal at work. Went named it auxin, from the Greek word for "to grow." That basic experimental logic — separate the source from the response, and see what moves between them — is still how a lot of plant hormone research works today.
Types of Plant Hormones
Auxins. The primary auxin in plants is indole-3-acetic acid (IAA), produced mainly in the apical meristem — the actively growing tip of the shoot — and in young leaves. Auxins control cell elongation and promote the development of roots and shoots, and they're central to gravitropism, the plant's response to gravity: when a stem is oriented horizontally, auxin accumulates on the lower side, causing cells there to elongate more than cells on the upper side, which curves the stem back upward. Auxin is also the hormone behind apical dominance, covered in its own section below.
Gibberellins. Gibberellins (often referred to by their most commercially used form, GA3) stimulate stem elongation, seed germination, and — in certain species — flowering. They're produced in the apical meristem, young leaves, and developing seeds. This is the hormone responsible for the dramatic stem stretch you sometimes see in a plant reaching for light, and it's also released naturally during seed germination to help mobilize stored food reserves in the seed.
Cytokinins. Cytokinins promote cell division and the development of lateral buds, and they're produced primarily in root tips, as well as in developing seeds and fruit. They work in direct opposition to auxin in several important ways, which is exactly why the balance — the ratio — between the two hormones matters more than either one in isolation.
Abscisic Acid (ABA). ABA regulates the plant's response to stress, including drought and cold temperatures, largely by triggering stomata (the pores on leaves) to close and conserve water. It also plays a central role in seed dormancy, effectively telling a mature seed to stay dormant until conditions are actually favorable for germination.
Ethylene. Unlike the others, ethylene is a gas. It regulates fruit ripening, flower senescence (aging), and abscission — the shedding of leaves, flowers, or fruit. Ethylene is also the science behind the old saying that "one bad apple spoils the bunch": a ripening or damaged fruit releases ethylene gas, which then accelerates ripening (and eventual spoilage) in nearby fruit exposed to it.
Newer Hormones Worth Knowing
Plant science has identified several additional hormone classes beyond this classic five, and a couple are directly relevant to the kind of growing and propagation work we cover on this site:
Strigolactones. These are directly involved in regulating branching, working alongside auxin to suppress lateral bud outgrowth — meaning apical dominance isn't just an auxin-and-cytokinin story, strigolactones are part of that conversation too. They also stimulate beneficial relationships between plant roots and mycorrhizal fungi in the soil.
Brassinosteroids. Steroid-like hormones that promote cell division, support vascular tissue development, and improve a plant's resilience to environmental stress.
Jasmonates and Salicylic Acid. Both are heavily involved in plant defense — jasmonates ramp up in response to physical damage like insect feeding, while salicylic acid activates defenses against disease-causing pathogens. Neither is something a home grower typically applies directly, but they explain a lot of what's happening internally when a plant reacts to being attacked.
How Hormones Actually Move Through the Plant
It's worth being clear about something that trips people up: not every hormone travels the same way. Auxin moves in a specific, energy-requiring, cell-to-cell process called polar transport, generally flowing downward from the shoot tip — this directionality is exactly why apical dominance works the way it does. Other hormones, including cytokinins moving up from the roots and ABA moving from roots to shoots during drought stress, travel more passively through the plant's actual plumbing: the xylem, which moves water and dissolved minerals upward from the roots, and the phloem, which moves sugars produced in the leaves to wherever they're needed. Both of these transport tissues are produced by a thin layer of actively dividing cells called the vascular cambium — the same layer that has to physically align between a scion and rootstock for a graft to succeed, and the same tissue that has to generate new cells (callus) at the base of a cutting before it can root at all. That's genuinely its own deep, important subject in plant anatomy — how water and nutrients actually move through a stem, and why the cambium layer matters so much for grafting and propagation — and it deserves a dedicated article of its own rather than a quick mention here. Look for that as a companion piece to this one and to the stem cutting anatomy article.
Apical Dominance: What's Really Happening When You Top a Plant
This deserves its own close look, since "topping" or "pinching" a plant to encourage more growth or fruiting is common advice that's rarely explained.
Auxin produced at the shoot tip moves down the stem and keeps the axillary buds below it dormant — this is apical dominance, and it's really governed by the ratio between auxin (highest near the tip) and cytokinin (arriving from the roots). A high auxin-to-cytokinin ratio keeps lateral buds suppressed; a low ratio lets them break dormancy and grow. When you remove the terminal bud — by pinching, topping, or pruning back a leader — you remove the main source of that suppressing auxin. Auxin levels in the stem drop, the ratio tips in cytokinin's favor, and the dormant lateral buds below the cut are released to grow.
This is genuinely two different practices with two related but distinct purposes, and it's worth telling them apart:
Topping a young plant to build more fruiting wood. Pinching the growing tip of a young pepper or basil plant, or making a heading cut on a young fruit tree during formative pruning, releases several lateral buds at once instead of just one leading shoot. Over time, that means a bushier plant with more branches — and since flowers and fruit often form on that branching structure (think of the fruiting spurs we've covered on apple and pear wood), more branches generally means more eventual fruiting sites. This is the mechanism behind almost all formative pruning of young fruit trees and shrubs.
Topping a mature, actively fruiting plant late in the season. This is a different situation, and the benefit is more about resource allocation than branching. Late-season topping of an indeterminate tomato plant, for example, stops the plant from continuing to pour energy into new vegetative growth and new flowers that have no realistic chance of ripening before frost, redirecting the plant's limited remaining photosynthate toward sizing and ripening the fruit that's already set. The hormonal mechanism (removing the apex, dropping auxin and gibberellin-driven elongation at that point) is related, but the practical goal here is finishing what's already there rather than encouraging more branches.
Using Plant Hormones to Control Growth and Development
Plant growers can put this knowledge to work in several concrete ways:
Pruning and Pinching. As covered above, removing a terminal bud doesn't so much "produce more" auxin and cytokinin as it removes the auxin suppression that was holding lateral buds back, letting the existing cytokinin from the roots do its job — promoting branching and, over time, more fruiting wood.
Gibberellin Applications. Commercial GA3 applications can stimulate stem elongation, useful for producing longer stems in cut flowers, and are also used commercially to enlarge seedless grape berries and to accelerate uniform germination in malting barley.
Ethylene Treatments. Ethylene gas, or the ethylene-releasing compound ethephon, is used commercially to ripen fruit like bananas and tomatoes quickly and uniformly after harvest — the same basic chemistry behind the old trick of putting an unripe fruit in a paper bag with a ripe banana to speed it along at home.
Abscisic Acid Treatments. ABA-based treatments can help induce or maintain seed dormancy, useful for extending seed storage life, though ABA products are less commonly available to home growers than synthetic auxins or gibberellins.
Rooting Hormone. Commercial rooting hormone products are almost always synthetic auxins — most often IBA (indole-3-butyric acid) or NAA (naphthaleneacetic acid) rather than natural IAA, since these synthetic versions are more stable and effective at stimulating root formation on a fresh stem cutting. This is directly relevant to the propagation work we've covered elsewhere on the site: dipping a cutting's base in rooting hormone before sticking it in a rooting medium meaningfully increases success rates, especially for harder-to-root woody plants.
A Word of Caution
Synthetic plant hormone products are genuinely useful tools, but several of them — particularly synthetic auxins used at higher concentrations — are regulated as plant growth regulators or even herbicides, and following label instructions exactly matters both for your plants and for safety. More is not better with these products; overapplication is one of the most common ways home growers accidentally damage or kill the plants they were trying to help.
Conclusion
Plant hormones play a critical role in regulating plant growth and development, and understanding how they interact — auxin against cytokinin, the timing of gibberellin and ethylene, the protective role of ABA — gives you real, practical leverage over how your plants grow, branch, root, and ripen. Used thoughtfully and according to label instructions, that knowledge turns pruning shears and a bottle of rooting hormone into precise tools rather than guesswork.
