My initial guess is that this is a variety of nettle, however it does not match any images I was able to find online. It grows “wild” in my backyard garden, becomes 6 or 7 feet tall by the end of summer, is very prolific, has insignificant flowers, is much loved by pollinators, and sheds very prolific seeds. The seed pods are much barbed, while the stock is multi-sided.
Your plant has had us a little stumped!
Without a flower it can be remarkably difficult to identify plants, as their leaves alone are often reminiscent of many different species. From your very detailed description, though, we think that the plant in your garden is Motherwort, Leonurus cardiaca L. Motherwort is a member of the mint family and is categorized as a weed in Ontario. This plant’s stem, like all mints, is squarish and hairy. Its basal leaves have three points, while its upper leaves have five, and in its non-flowering stage, like your photo, its leaves resemble maple leaves. Flowers are clustered, and are pink to lilac in colour, often with furry lower lips. The calyxes of the flowers become spiny bur-like clusters on mature and dry stems. Motherwort is common throughout southern Ontario in yards, waste places, fence lines and roadsides. Its stem can reach heights of 70 inches, matching your description. Motherwort is considered to be a medicinal herb, and herbalists describe it as a gentle sedative that helps calm the entire nervous system and is prescribed for heart palpitations (hence the latter part of its Latin name – the first part means “lion’s tail”). It is also used by women to manage PMS and menstruation. Motherwort is considered to be an edible herb, eagerly sought by foragers.
Here is a website that provides some good photos of its flowers and burs – as well as many busy pollinators: https://toronto-wildlife.com/Plants/Mint_family/more_motherwort.html You should be able to tell instantly if this is your plant. If Motherwort is indeed what is growing in your garden, you may wish to read this complete description provided by the Ontario Ministry of Agriculture, Food and Rural Affairs:
It is interesting that you remarked on the similarity of the leaves to nettles. The stinging nettle is of a different family entirely, but the deadnettle is a member of the mint family, like Motherwort.
We would be keen to know if our identification is correct. Thanks so much for reacquainting us all with this fascinating plant.