Weed profile – Broomrape (Orobanche)

A brown stalk with purple-brown, almost snapdragon-shaped or sage-shaped flowers radiating around it sits amongst lush green broadleaf weeds

Orobanche flower stalk with open flowers, rising up from amongst the weeds hosting it. Before the flowers separate and open it looks rather phallic and odd-shaped.

This weed amuses me immensely. It’s a curious thing. Note the colour – it looks dried out even though it isn’t, there’s not a speck of green. That’s because it has no chlorophyll – none of that green stuff that plants use to create energy from sunlight. It gets all its energy from parasitising other plants. There are maybe 150 species or thereabouts in the Orobanche family, mostly called something-or-other broomrape, mostly native to Europe and the Middle East, almost all from the Northern Hemisphere, and all interesting shades of brown, yellow and purple. We have a couple of kinds that have made their way to Australia plus one native species. The one we usually see as a weed is Orobanche minor from Europe which is pretty commonly distributed around the SW of WA. It spends most of its life underground but puts up a flowering stalk around this time of year.

A thick bulbous brown stalk with orange roots is entangled with the white roots of another common garden weed. The roots join white to orange almost seamlessly, as if one plant.

Look carefully and you will see where the orange orobanche roots join with the white roots of its host, one of the Asteraceae weeds that is noticeably smaller than the others growing around it.

The Orobanches are pretty cool. The seeds are triggered to germinate by chemicals produced by the roots of their host plants. Once they sprout, they start growing towards and along the strongest chemical trail in the soil around them – imagine the growing tip “tasting” the soil as it goes. If they successfully find the root of one of their host plants, they attach to it with haustoria and start siphoning off its energy for themselves. Many of the Orobanches are such specialised “tasters” that there’s only one species of host they can find and attach to. Orobanche minor is one of the rare generalists. It can use a surprisingly wide variety of plants as hosts. It does tend to favour the Fabaceae or pea family though with the Asteraceae or daisy/dandelion weeds as a close second. I usually find it on asteraceous weeds such as the Common Sow-thistle. So in the home garden it’s not too much of a pest – it’s more likely to be attacking and draining other European-origin weeds than the garden flowers. And it does a good job of draining them.

It becomes more of a problem in crops though, both broadacre and horticulture. While this species is considered merely a “minor weed”, an infestation of Orobanche can destroy a clover crop, affecting the whole rotational system, or drop yield in chickpeas and lentils by up to 50%. This species can drain a lettuce or bean crop. There is even one species that will drain marijuana. The ways to remove it start with trying to keep it out in the first place. Its seeds are numerous and among the tiniest in the world -like dust!- so once in it’s hard to get out but clean cultivation and getting clean seed isn’t too hard. Once in, it’s about trying to stop it germinating and/or setting seed. I’ve also seen the suggestion of planting in intervening years a crop that will cause the Orobanche to germinate but which it can’t host from – such as some of the nightshades – but that depends on which species of Orobanche you’re dealing with and it looks like we don’t have too many to choose from here. The one Australian native species doesn’t affect commercial crops at all.

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