Cutting Failure in a Small Nursery Batch
A TAFE nursery unit has taken 400 softwood cuttings of a lavender cultivar and 300 semi-hardwood cuttings of a native correa. The lavender cuttings were taken at midday, placed in a bucket of water in sun for two hours, then struck into a 50/50 peat-perlite mix in trays under mist. The correa cuttings were taken early morning, kept in a cool esca, wounded and dipped in 3000 ppm IBA talc, and struck into the same mix in a separate bay with bottom heat at 22 °C. After three weeks, 70% of lavender cuttings have collapsed with black stem bases, while correa callusing is poor and only 15% have rooted. The mist bay runs 6 seconds every 8 minutes during the day. Night temperatures in the glasshouse drop to 8 °C.
1. Explain the two failures separately and identify the most likely causes.
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The lavender collapse is classic basal rot from prolonged wet foliage and warm, still conditions. Cuttings sat in sun in a bucket for two hours, which stresses soft tissue and encourages bacterial and fungal entry. The 6-second mist every 8 minutes is likely too frequent for softwood lavender, keeping the medium saturated and the stem bases constantly wet; combined with 8 °C nights and poor air movement, this favours Botrytis and water moulds. The correa failure is different: poor callusing with only 15% rooting suggests the cuttings are being kept too cool at the base or the bottom heat is not reaching the medium effectively, and the 3000 ppm IBA talc may be too strong for a semi-hardwood native, causing stem burn rather than promotion. Correa also benefits from firmer wood and good drainage; the peat-perlite mix may be too moisture-retentive under mist. Night temperatures of 8 °C slow root initiation in both species.
2. Propose revised propagation protocols for both species.
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For lavender: take cuttings early morning, keep cool and shaded, never in standing water, and strike within an hour or two. Use a free-draining mix, reduce mist to short bursts only as needed to prevent wilting, increase air movement, and maintain night temperatures above 12–15 °C. Consider a light IBA treatment (1000–1500 ppm) rather than none or excessive hormone. For correa: use semi-hardwood material, keep the base warm with bottom heat verified at the medium (not air), reduce IBA to around 1000–1500 ppm or try a liquid dip, and improve drainage. Reduce mist frequency and duration, and raise night temperatures. Set up a small trial: 50 cuttings per species across two hormone rates and two mist settings, record callusing and rooting at weeks 3, 5 and 8, then adopt the best-performing combination. Keep records of stock plant, date, treatment and environment for every batch.