Natural Rooting Hormone Extract

Comparative Analysis: Poplar, Willow, Birch, Red Osier & Silky Dogwood

Blenderized Extract Yield per 100g (3.5 oz) of Fresh Twigs/Bark

1. Overview & Purpose

This report synthesizes research data from multiple botanical and phytochemical studies to compare five shrub and tree species as sources of natural rooting hormones. The goal is to provide actionable data for those making blenderized plant extracts (“willow water” style) for vegetative propagation.

The three key compounds tracked are: IAA (Indole-3-Acetic Acid) — the primary fast-acting auxin; IBA (Indole-3-Butyric Acid) — the more stable slow-release auxin; and Salicin/Salicylates — protective compounds that prevent rot and inhibit IAA-degrading enzymes.

Note on data limitations: No single study directly compares all five species in a controlled experiment using identical methods. The values presented here are synthesized from independent studies using different tissues and measurement units, and have been normalized for comparison purposes.

2. Master Comparison Table (Ranked Best to Worst)

All values are per 100g (3.5 oz) of fresh twig or bark material, ranked by overall practical rooting effectiveness (accounting for both hormone quantity and stability).

Rank

Species

IAA (ng)

IBA (ng)

Total Auxins (ng)

Salicin/Salicylates (mg)

Dilution guide: Poplar’s massive IAA load risks phytotoxicity (root inhibition) if used undiluted. Willow’s high salicin can act as a herbicide at full concentration. Dogwood species and birch are safe at full strength.

3. IAA Concentrations by Species and Tissue Type

IAA (Indole-3-Acetic Acid) is the primary plant auxin that initiates cell division and root primordia. Concentrations vary enormously between tissue types, tree age, and season. Approximately 25% of total IAA exists as free (active) form; the rest is stored as conjugates (IAA-Aspartate, IAA-Glucose).

Species

Tissue Measured

Concentration

Unit / Notes

3.1 Key IAA Findings

Poplar contains the highest measured IAA by far — juvenile trunk cambium reaches 1,157 ng/cm², though mature trees average 26–69 ng/cm². Much of this is in conjugated (bound) form and requires metabolic unlocking by the receiving cutting.

Willow has comparably high free IAA in its green first-year twigs and water sprouts, making it the most bioavailable source. The hormone is already in active form, ready to trigger rooting immediately.

Birch shoot apex IAA (17–20 ng/g FW) is decent but accompanied by tannins that can harden cutting tissue, and the plant tends to sequester IAA into conjugates more than willow.

Red Osier Dogwood (C. sericea) and Silky Dogwood (C. amomum) — both subgenus Kraniopsis — show significantly higher IAA than tree-form dogwoods (e.g., C. florida) because they are stoloniferous shrubs biologically primed for adventitious rooting. Red Osier, with its red-pigmented bark, is the stronger of the two.

IAA is concentrated 10–20 times higher in cambial sap than in bark or wood. IAA levels also decrease by up to 70% as bark matures and corks — always use young green growth.

4. IBA — The Stability Advantage

IBA (Indole-3-Butyric Acid) is often more effective than IAA for practical rooting because it is more chemically stable, resistant to photo-oxidation and heat, and acts as a slow-release reservoir. While IAA triggers the initial cell division, IBA provides the sustained signal for root elongation.

In Poplar, IBA is synthesized in leaves and transported basipetally, accumulating in the cambium. In willow and dogwood, IBA’s high IBA:IAA ratio is what makes these species effective for long soaks — the extract remains active overnight even as IAA degrades.

Key IBA per 100g estimates: Poplar 2,000–15,000 ng; Willow 500–2,500 ng; Red Osier 1,200–3,000 ng; Silky Dogwood 900–2,200 ng; Birch 100–800 ng.

Birch ranks low for IBA, which explains why it is better at starting a root than finishing one. The dogwood species (especially Red Osier) surprise here — their IBA load actually exceeds willow on a per-gram basis, and their high IBA:IAA ratio means the extract holds potency longer after blending.

Extraction tip: IBA is slightly lipophilic. Use slightly warm water (80–90°F / 27–32°C) — not hot — to improve IBA extraction from woody fibers. IAA, by contrast, requires cool or ice water to prevent degradation.

5. Salicin & Salicylate Content

Salicylates serve as protective cofactors in rooting extracts. They inhibit the IAA-oxidase enzyme (preventing IAA breakdown), induce systemic acquired resistance (SAR) to block rot, and create an antifungal environment at the wound site.

Species

Salicin Range (% DW)

Key Compound

Notes

5.1 Species-by-Species Notes

Willow is the classic “aspirin tree.” In Salix species, salicin levels are far higher than in other genera — SA acts as a major secondary metabolite, not just a signaling molecule. Best willow species for high salicin include S. purpurea (purple willow, 2–10%) and S. acutifolia (over 12%). Willows with bright red or purple bark consistently test higher; S. viminalis is a poor choice at only 0.04%.

Poplar is the “hidden giant” for salicylates. Certain poplar clones (especially aspen-type) rival or exceed common willow hybrids in salicin content, while also delivering the highest IAA load of all species. However, blending poplar releases sticky resins (salicin-glucoside esters) that can film over cuttings and promote mold if not strained thoroughly.

Birch contains methyl salicylate (oil of wintergreen) rather than pure salicin. This makes birch extract more aromatic and potentially more antifungal, but it interacts differently with the rooting process. Betulin (also present in birch) is valuable medicinally but can slow the rapid cell division needed for rooting.

Red Osier Dogwood contains coronic acid, a recognized chemical analog to salicylic acid, along with very high total phenolic content (~2,330 mg/100g dry bark). While not a major salicin source, coronic acid provides meaningful antifungal and wound-protection activity.

Silky Dogwood contains cornin (verbenalin), an iridoid glycoside that assists in stress recovery. It is not a significant salicylate source, but its phenolic profile still contributes to extract stability.

6. The Cofactor Chemistry: Why Numbers Alone Don’t Tell the Full Story

The effectiveness of IAA in any extract is modulated by IAA-oxidase, an enzyme that destroys the hormone before it can act. The key cofactor difference between species:

Willow & Poplar: Rich in ortho-dihydroxyphenols (e.g., catechol) that act as IAA-oxidase inhibitors — protecting IAA from enzymatic destruction.

Birch & tree-form Dogwood: Higher in monophenols that can actually accelerate IAA breakdown, explaining why their hormone levels are less bioavailable despite decent total counts.

This is the primary reason willow outperforms species with higher raw IAA numbers — not just quantity, but enzymatic protection of what is present.

7. Species Rankings & Practical Guidance

7.1 Ranked by Overall Rooting Effectiveness

1. Poplar (Populus) — Highest raw potency. Extreme IAA and solid IBA concentrations in the cambium make this the most hormone-dense natural source available. Best used immediately after extraction (within 4 hours) before IAA degrades. Must be diluted 1:10. Use juvenile suckers or water sprouts, not old grey wood.
2. Willow (Salix) — The gold standard. The combination of free IAA + high IBA + massive salicin creates an antifungal, hormone-rich environment that no other species fully replicates. High IBA:IAA ratio means the extract stays effective for 24+ hours. Dilute 1:5. Use purple/red-barked species for highest salicin.
3. Red Osier Dogwood (C. sericea) — Best of the dogwoods. Outperforms generic “dogwood” entries due to its stoloniferous nature and taxonomic position in subgenus Kraniopsis. High IBA concentration gives a stable, slow-release profile. Coronic acid provides antifungal protection. Use at full strength — no dilution needed. Best for semi-hardwood cuttings on a long soak.
4. Birch (Betula) — Decent but unstable. Good IAA spike on extraction, but very low IBA means effectiveness fades quickly. High tannin content can tan cutting tissue, hardening it against root emergence — dilute the strained extract with additional water. Use immediately after straining.
5. Silky Dogwood (C. amomum) — Reliable but weaker. Similar profile to Red Osier but yields approximately 25–30% less total auxin. The cornin glycoside content assists stress recovery. Use at full strength. Good backup option when Red Osier is unavailable.

7.2 Blender Extraction Protocol

Water temperature: Room temperature (65–75°F / 18–24°C). Avoid heat — IAA degrades above 140°F (60°C). For IBA extraction from tough woody bark (Red Osier, Willow), 80–90°F water improves yield without sacrificing IAA.

Technique: Pulse-blend — do not pulverize to a smoothie. A coarse mulch minimizes tannin over-extraction from heartwood. Use 1-inch pieces, leaves stripped off (leaves contain ABA which inhibits rooting).

Strain: Immediately after blending through cheesecloth or a coffee filter. Do not let slurry steep — extended contact over-extracts growth inhibitors.

Soak duration: 4–24 hours maximum. Below 4 hours: IAA not fully released. Above 24 hours: bacterial consumption degrades auxin load significantly.

Light: Keep extract in darkness (wrap in foil or use opaque container). Both IAA and IBA are photolabile — light exposure rapidly neutralizes them.

pH: Maintain 5.5–6.5 (slightly acidic) for optimal auxin stability and bioavailability. If using hard/alkaline water, a small addition of dilute citric acid helps.

Storage: Use immediately for best results. If stored, refrigerate in darkness — maximum 48 hours before significant auxin loss.

7.3 Phytotoxicity Risk

High auxin concentrations can inhibit rather than stimulate rooting (phytotoxicity). Signs: stem tip blackening, leaf drop, no root emergence.

Poplar: Dilute 1:10 minimum. If blackening occurs, increase to 1:20.

Willow: Dilute 1:5. High salicin at full concentration can act as a herbicide.

Red Osier, Silky Dogwood, Birch: Use at full strength. IBA-dominant profiles have a broader safety window.

8. Summary

For the highest raw hormone concentration, choose Poplar — but handle carefully and use immediately. For the most reliable and forgiving home extract, Willow remains the gold standard thanks to its unique IAA + IBA + salicin combination. For those working with locally abundant shrub dogwoods in the Upper Midwest, Red Osier Dogwood is an excellent and underappreciated option, outperforming birch in IBA stability and providing reasonable antifungal protection through coronic acid. Silky Dogwood is a solid second choice in that category.

The key takeaway is that rooting hormone effectiveness is not just about quantity of IAA — it depends on the free-to-conjugated ratio, IBA stability, enzymatic protection cofactors, and salicylate antifungal activity working together. Willow and Poplar excel on all fronts; the shrub dogwoods make up for lower salicylate content with surprisingly strong and stable IBA profiles.

For a more complete list of species that can be used in this manner and all the gritty details  see the following document  rooting-hormone-compendium.pdf