Spekboom rooting — testing the stem-damage hypothesis.

In a previous post (To the root of the problem...), I highlighted that the roots of planted truncheons invariably only grow from the very base of the stem — thus, rooting takes place 15-20 cm below the soil surface where the stem has been cut. This was based on a few field observations that have been bolstered by digging up many more plants (must be over 50 by now, all in very different environments). In that same post, I suggested that damage to the stem was required for roots to develop. Together with Nicholas Galuszynski, we had an Honours student conduct a series of treatments to determine if this stem-damage for rooting was correct. Unfortunately, the Honours student left with all the data (another lesson on data management for me), so I'm left with a few photos — but the results are, nonetheless, quite clear.

We had five treatments:

1. Control, where the truncheon was cut and planted with no addition damaging.
2. Vertical slits, where two shallow vertical slits were cut (quite shallow: <0.5 cm) for ~10 cm length-ways along the stem ending at the base where using the sharp blade of a pair of secateurs.
3. Horizontal slits, where a number of horizontal slits were cut (also shallow) cross-ways across the stem. 
4. Peeling, where the sharp secateur-blade was used to shallowly peel (or skin) the outer bark, and,  
5. Deep gouging, where deep cuts were gouged into the stem.

And the overall results are summarised below with photos of single replicates.
1. Control: no roots along the stem

2. Vertical (or longitudinal cuts): no roots along the stem

However, there was some swelling at the base with more (visually assessed) root biomass.

3. Horizontal (or perpendicular cuts): no roots along the stem

4. Scraping: no roots along the stem

5. Vertical (or longitudinal cuts): hooray! roots along the stem

Rooting was consistent across stems with gouges, but not every gouge developed roots. Nonetheless, those that did not develop roots still healed over.

So, the deep gouging makes a substantial contribution to stimulating rooting. The great thing about these results is that they've been replicate by an independent experiment!

Thankfully, Yondela Norman — an Honours student at Rhodes University — conducted a completely independent and different experiment on rooting (at the same time as we were!) and also found that the deep gouges substantially promoted root growth. So, my predictions in this earlier blog were correct. It's great when the data actually supports a hypothesis.