Making difficult Scotch broom babies

Author: Katrina Cousins


Earlier last year I spent 4 eventful months at the Silwood Park campus of Imperial College, in southern England, conducting a field cross pollination experiment on Scotch broom (Cytisus scoparius Fabaceae).  The main aim of this experiment was to test for heritability of seed size and maternal effects on this plant, which is federally listed in Australia as a weed of national significance. My personal aim was to successfully produce Scotch broom babies whilst keeping warm.

Specimens for this experiment had to be planted in the UK four years ago due to strict quarantine restrictions on growing Scotch broom in Australia. Experimental plants were from native or invader  populations initially studied by Dr Yvonne Buckley (Buckley et al. 2003). Her 2003 study showed that seed size mass was heavier in invader populations compared to native.  The study also concluded that if seed size was in fact heritable, and larger seeds mass was an indication of increased seedling survival in invader populations, then there may be selection for larger seed size in global introductions.


Both Yvonne and I, in several layers of winter clothing, commenced preliminary experiment trials in the spring of April 2010 to establish the best emasculation technique to cross pollinate multiple Scotch broom plants (Simpson et al. 2005, Paytner et al. 2010). This video link highlights the final modified emasculation technique adopted for this heritability experiment.

By mid May 2010, with half of the experimental crosses completed, Silwood Park experienced the latest frost on record (Mick Crawley pers comm). The heritability experiment had to recommence in mid May due to  severe frost damage of emasulcated buds. The last emasculation procedure was finally completed early June 2010. By now, England was in the grips of World Cup fever with an unseasonal and fabulous summer developing. Therefore, I escaped to France (photos 1 & 2).

Photo 1.                                                 Photo 2.

Working hard whilst travelling in France (Photo 1. Looking for broom in the Provincial vineyards , Photo 2. French broom in Avignon)

During my absence for some well deserved R&R, the hot and dry weather in southern England appeared to have accelerated pod formation and seed development in most emasculation experiments.  As a result, all babies (technically referred to as Scotch broom experimental seeds) were collected by the end of July 2010.  The various stages of the heritability experiment are shown in photos 3-5.

Photo 3.                                   Photo 4.                              Photo 5.

Stages of the heritability experiment (Photo 3. Early budding and bagging, Photo 4. Pod formation after experimental crossing & Photo 5. Experimental seeds in stage of late development and collection)


Over 30 reciprocal crosses were successfully completed in this heritability experiment and, to date, all babies have been weighed.  We are in the process of analysing results, which will be completed this year.


Chief Investigator – Dr. Yvonne Buckley

Researcher – Katrina Cousins


Buckley, Y.M., Downey, P., Fowler, S.V., Hill, R., Memmot, J., Norambuena, H., Pitcairn, M., Shaw, R., Sheppard, A.W., Winks, C., Wittenberg, R. & Rees, M. (2003). Are invasives bigger? A global study of seed size variation in two invasive shrubs. Ecology 84(6): 1434-1440.

Paynter, Q., Main, A., Gourlay, A.H., Peterson, P.G., Fowler, S.V. & Buckley, Y.M. (2010). Invasion of varroa mite, Varroa destructor, can enhance biological control of Scotch broom Cytisus scoparius in New Zealand. Journal of Applied Ecology 47: 309-317.

Simpson, S.R., Gross, C.L. & Silberbauer, L.X. (2005). Broom and honeybees in Australia: An alien liaison. Plant Biology 7(5): 541-548.


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