Biostimulants are valuable tools in horticulture. They can improve rooting, enhance resilience, and support stress recovery. But just like with fertilizers or crop protection products, the right dose and the right combination are crucial.
In a recent case, callus tissue developed in young pot roses. Callus is disorganized, undifferentiated plant tissue — essentially the plant “switching on” regeneration mode. While this may sound beneficial, in practice it can lead to poor root development, growth abnormalities, and weaker plants.
After ruling out pathogens such as Agrobacterium tumefaciens, the focus turned to cultural practices and inputs. The analysis showed:
- Electrical Conductivity (EC) was relatively high for rooting cuttings, limiting proper root initiation.
- Recirculated irrigation water inadvertently carried multiple rounds of biostimulants to young plants.
- Hormone-rich inputs such as certain seaweed extracts (rich in cytokinins) posed a moderate-to-high risk of callus formation, especially when combined with rooting powders or other stimulants.
Other biostimulants like vermicompost or amino acid mixes had low risk, but the synergy of multiple stimulants together created “too much of a good thing.”
Key takeaways:
- Seaweed extracts are powerful, but overdosing or doubling up with similar products can unbalance auxin–cytokinin ratios, triggering callus instead of healthy roots.
- Combining stimulants without clear strategy can cause antagonistic or unpredictable effects.
- Safer, complementary combinations include worm compost + amino acids (for soil and nutrition) or chitosan + amino acids (for stress management).
- In propagation, choose either a rooting powder or a hormonal biostimulant — rarely both.
The lesson? Biostimulants are not “the more, the better.” Used thoughtfully, they build plant strength. Used excessively or in the wrong mix, they can compromise the very growth they are meant to support.