Sorry, it was a namespace issue.
Since the 'plants' urls are included in the main urls.py, the solution is:
{% url 'plants:detail_view' cat=category_slug slug=plant.slug %}
I'm sure that Samuele Mattiuzzo would have caught it had I said that the url file was plants/urls.py
but I left out that important detail.
Thanks for your help, anyway.