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Our garden in Christchurch New Zealand has been invaded this summer by what I have in my profound ignorance named stalky things. These weeds have a single, strong central stalk. No hint of flowers has been observed. Their roots are shallow: the largest specimen I have removed, shown below, has no more than 10cm of root supporting more than 1m of growth.

I'd be grateful for any hints toward identification. I'm happy to provide more information - I've never asked such a question before, so I may well have left out something obvious and critical.

Here is an uprooted 1.2m specimen:

enter image description here

And here are two smaller examples in the garden, the larger 30cm:

enter image description here

Added 14 February 2020:

After waiting until late in our summer for flowering of the plant, this has begun at the very tip of the largest specimens (>1m). Here is the detail:

enter image description here

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    That's a real winner for a weed. Good question – kevinsky Jan 17 at 16:14
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Answer edited to reflect new research

I think you might have horseweed AKA Canadian fleabane (Conyza canadensis). This plant can grow to nearly 2m high, blooms late in our summer, and flowers only from the top, which matches your weed since you haven't seen any flowers yet. I've checked the Invasive Species Compendium, and this plant has indeed been introduced into NZ.

It may also possibly be a garden escapee such as evening primrose (Oenothera species, possibly O. biennis), although I'm not sure that you have that genus naturalized or native in NZ (O. biennis is a North American native that can grow 2m+ high, flowers in late summer, and also blooms only at the top of the plant).

Here's one site from Illinois that shows some photos of O.biennis for a quick comparison.

A close-up photo of the living plant's stem - particularly where the leaves join the stem - would be helpful for the ID.

  • +1 Thanks for your research. I decided to await developments later in the summer. I have added an extra image of the development at the tip of the plant. Looking at your links with an inexpert eye, Conyza canadensis looks like a very good match. I'll leave the question for a day or so in case anyone else has something to offer, but it looks like I'll be accepting your answer. – replete Feb 14 at 3:59

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