A quick Google search of the above question returns many anecdotes of growers having success with trimming the leaf tips of cannabis clones. However, there is very little peer-reviewed research that addresses this question. When cloning cannabis plants, growers use a number of techniques to increase the rate of root generation and number of roots produced. Ideally, a clone will produce many roots as quickly as possible so that it may move on to have a successful vegetative and flowering period. Here, I’m going to present the results of one study published by a Canadian researcher that addresses this question. The research looks at the effect of (1) leaf tip removal, (2) leaf number, (3) use of rooting hormone, and (4) location of stem on the mother plant on the success of root growth. A link to the original article is at the end.

What are some Common Reasons for Cutting Leaf Tips?

Cutting leaf tips
Reducing leaf number
Using synthetic rooting hormone
Reasons In Favour

Leaves provide surface area for evapotranspiration which might negatively affect rooting success (Davis & Potter, 1989). Too many leaves may crowd propagation trays.

Similar to above: high evapotranspiration may negatively affect rooting success.

Organic rooting hormone may be favoured by some growers and consumers who value organic and pesticide-free cannabis.

Reasons Against

Leaves are a source of nutrients for clones and cutting leaf area may reduce nutrients (Ofori et al., 1996). Leaves are also a source of natural rooting hormone.

Similar to above: with more leaves comes more nutrients and thus more roots.

Synthetic rooting hormone is well-documented to improve rooting success by supplying auxins (Hartmann and Kester, 2002).

Experiment

Clones with 3 leaves were cut and rooted in Pro-Mix growing medium (peat moss-based). Organic fertilizer was supplied. Relative humidity was kept between 80% – 95%; temperature was 24°C; light intensity from fluorescent bulbs was between 50 – 115 µmol·m-2s-1 for 18-hour photoperiod.

The following combinations were tested:

  • Leaves left uncut vs. 1/3 of leaf tip cut
  • Three leaves vs. two leaves (1 leaf removed) per clone
  • Stem dipped in synthetic rooting hormone vs. organic willow extract rooting gel
  • Clone taken from top of plant (> node 10) vs. bottom of plant (< node 10)

Rooting success: Clones were evaluated for the presence or absence of roots. Trays were scored based how many plants had roots vs. no roots.

Root quality score: Plants that had lots of roots were considered high quality and plants with few roots were considered low quality.

leaf-tips-rooting-quality

Results

cut-leaf-tips-rooting-success
cut-leaf-tips-rooting-quality
What resulted in higher rooting success?
  • Clones with UNCUT leaf tips had higher rooting success than clones with CUT root tips.
  • Clones dipped in synthetic rooting hormone had higher rooting success than clones dipped in organic willow rooting hormone
  • Neither leaf number OR position of cutting from mother plant had an impact on rooting success
What resulted in higher quality (ie. more) roots?
  • Clones with 3 leaves had more roots than clones with just 2 leaves
  • Clones dipped in synthetic rooting hormone had higher root quality than clones dipped in organic willow rooting hormone
  • Neither leaf cutting OR position of cutting from mother plant had an impact on rooting success

Take-Home Messages

The type of rooting hormone used strongly impacts the success and quality of rooting in cannabis. Cutting leaf tips reduces rooting success rate and having three leaves on the clone results in higher root quality than clones with just two leaves. So if you want your clones to develop many roots quickly, take cutting from either the top or bottom of the mother plant that have at least 3 full leaves. Do not cut the leaves, but dip them in synthetic rooting hormone. If you find that your propagation tray is too dense or humidity is too high, it’s better to reduce leaf number than cut the leaf tips.