Weed of the Week - Orange Hawkweed
Orange Hawkweed (Hieracium aurantiacum AKA Pilosella aurantiaca)
Prohibited Noxious Under Alberta's Weed Control Act
Submitted By Kelly Cooley, CoolPro Solutions
Weed Of The Week took a hiatus last week, so this week we will catch up with the invasive hawkweeds – those with orange and yellow flowers. First up is Orange Hawkweed - a species tough to miss when in flower – but nearly unnoticeable otherwise. Seeds germinate in the spring and form densely white-haired lance-shaped leaves in ‘rosettes’ close to the ground, out of which solitary and leafless black-haired stems emerge. All leaves and stems yield a milky white sap if broken open. These stems eventually yield a cluster of black-haired buds that open in late June to early July to reveal the brilliant orange flowers (see photos). While these small flowers are undeniably attractive, this is a terribly invasive species, which checks all the boxes for aggressive spread. This creeping perennial tends to rapidly spread via rooted clumps tied together laterally below ground (roots) and above ground ‘runners’ (stolons). In addition, Orange Hawkweed’s flowers can either wait for pollinators, or asexually reproduce without pollination. This combination of reproductive strategies allows the species to produce as many as 40,000 seeds in a 1 metre square patch!
Orange Hawkweed is capable of invading and completely dominating desired native or tame vegetation in pastures, hay fields, turf, and even shaded tree & shrub areas. This species has marched through the province of British Columbia and is aggressively spreading east of the Continental Divide into Alberta. While many herbicides are capable of controlling Orange Hawkweed, once the plant has flowered, herbicides are generally ineffective in preventing viable seed production.
Landowners who spot Orange Hawkweed on or near their land should take great haste to bring the infestation under control before it spreads using its competitive advantages. Learning to spot the plant’s densely hairy rosettes will help in this regard, allowing for control measures prior to flowering. More information on this aggressive invader can be found in the Orange Hawkweed fact sheet from the Alberta Invasive Species Council, and you can report this invader yourself using their free EDDMapS application on your mobile device. For local Orange Hawkweed control options, please contact our Agricultural Fieldman at 403-339-8741.
Kelly Cooley, CoolPro Solutions
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