Plantlets grown in a culture vessel live in a very different world from a greenhouse. Hardening, also called acclimatisation, is the stage where most tissue culture projects gain or lose their margin.
Why plantlets are fragile
- They grow in near-saturated humidity, so their leaf cuticle and wax layer are thin.
- Stomata may not close properly, so they lose water quickly in drier air.
- Roots formed on agar may be poorly adapted to substrate and to taking up water.
- Many plantlets get part of their energy from sugar in the medium and must switch to full photosynthesis.
Step by step
- Prepare the plantlets. Choose well-rooted plantlets with healthy leaves. Remove the plantlets gently and rinse off all agar, which would otherwise feed fungi and bacteria.
- Use a clean, well-drained substrate. Mixes such as cocopeat with perlite or vermiculite are common choices; they should be low in nutrients and free of pathogens.
- Start with very high humidity. Cover trays or use a fogging chamber, and open the cover gradually over about two to four weeks, longer for sensitive species.
- Give gentle light. Shade of roughly 50 to 75 % at first, increasing light as new leaves form.
- Keep temperature steady. Many species do well around 25 to 28 °C.
- Water carefully. Keep the substrate moist but never waterlogged, and avoid wetting leaves late in the day.
- Begin feeding lightly. Introduce dilute nutrients once new growth appears.
What lifts survival
- A dedicated hardening area, not a corner of the greenhouse.
- Controlled misting or fogging linked to humidity sensors, so conditions do not depend on who is on shift.
- Records of each batch, so poor results trace back to the cause.
- Strict hygiene: clean trays, clean water and a fungicide or biological plan suited to the crop.
Survival depends heavily on species, plantlet quality and conditions. Read our guide to setting up a tissue culture lab, see the lab setup service, or learn how we design hardening houses in our greenhouse service.