Recently, I’ve had to move the heat mats from the tropical glasshouse into the propagation area for Banskia cuttings (something I hope to discuss soon). During this rearranging it became clear some of the Cacti in the tropical house were rotting, so I decided to propagate and replace them.
Cacti are plants in the Cactaceae family, which encompasses around 130 Genera and 1750 species. Cacti have very specific features that differ from other plants. One of those key features is the presence of structures called Areoles. Areoles typically appear as small knobs on the surface of the plant, making them easy to recognise. Biologically, they are modified and reduced branch structures that give rise to flowers and often spines.
Another consistent feature of the Cactaceae family, and one that particularly affects propagation, relates to water absorption. Unlike most plants, Cacti cannot directly absorb water through their stems. This means they will not root in the presence of water. So, when attempting to root Cacti, the potting medium should be moist and then should not be watered until root formation has occurred. The cutting will try to produce roots in search of water whether the medium is dry or saturated, but more moisture only makes the wounded cutting and new tissues more vulnerable to rot. It’s a situation where some level of neglect is favourable!
We use another technique to reduce rot on the new cuttings called “callusing.” Before the cuttings are placed in a medium, they are air dried. The dried stem plates help prevent fungi from entering and rotting out the fleshy stem.
The image above shows the cuttings callusing, this often takes 4-7 days in dry air. It’s worth being aware that Opuntias often take 9-12 days. In the image above, you’ll notice I’ve cut some of the segments of Hylocereus in half: it’s often best to cut full segments at the joints, but I was willing to take a risk here and experiment. I want to see if the half segments actually root better or worse than full segments.
Once callused, the cuttings are inserted into pots with a moist medium. I used a medium consisting of 20% coir, 20% tufa and 60% sieved perlite (perlite is sieved to remove finer dusty particles). The medium has a high air porosity to make root formation quicker. As I mentioned, I won’t water the cuttings until they begin to form roots. Hopefully in a few weeks I will have some new cacti!