Was this ever a vegetable garden? Or have you used home made compost there?

Could it be a Gerbera Daisy?

Spinacia oleracea

That’s why I asked those questions ....

I knew that

I ignored them. Spinach you say.

Looks like some healthy spinach to me as well.

Strangely I didn't have any seeds of it and it grew by itself. If I let it grow will it produce seeds?

It should.

Might that bed once had vegetables in it?

See above....

I never grew this vegetable in my garden.

Have you planted any other seeds or plants there recently? Might it have been a stray seed mixed in?

Excellent, I bought marigold from market. Now will they bloom and can I harvest there seeds?

Spinach will flower and produce seed if left. But the blooms are uninspiring, seed takes a while to ripen and meanwhile you have a scruffy two foot tall stem of dirty green, then brown, bobbles sticking up out of your flower bed.

If you want to grow spinach a pack of seed of a named variety costs very little.

Looks like Basella alba to me.

It does a bit. But in Toronto?

I considered Basella. Though you can't see a full leaf in the pic, these leaves seem more oblong then heart shaped and judging by the leaf sizes there should be a long stem protruding from them by this time, unless it's covered up, but I doubt that one leaf folded over would mask the stem growth. The glossiness reminds me of Basella. A taste test would not work. Maybe a retarded Basella alba with non typical growth? I've grown Basella that did weird things.

Leaves in photo appear to be growing from a single basal area. Malabar spinach doesn't grow like that, IMPE.

I think it's interesting that both spinach and Malabar Spinach are in the same order of the pinks Caryophyllales, but in 2 different families, yet their leaves are so similar, when leaf forms in that order have radiated into genera like Drosera and Dionaea.

Spinacia oleracea

Basella alba

Some botanists have separated the Chenopodium subfamily Chenopodoideae into it's own family Chenopodiaceae. I see no reason to waist time and effort cultivating Spinacia oleracea when there is so much free Lambs Quarters, Chenopodium alba free for the taking. A bag of spinach from the store is cheap and easy.