Annual sow thistle (Sonchus asper)
A prolific annual from Europe, it has ovate, shiny cotyledons (the first two seed leaves) with a mid-vein on the underside. Each plant can produce 26,000 seeds in one season that are viable for up to eight years. Remove with a hoe or garden fork; wear gloves.
Common burdock (Arctium minus)
A biennial from northern Europe, it has elliptical, dull green cotyledons with a purplish green stem below. Mainly found around the garden perimeter as an established plant, it has to be removed with a garden fork to get the entire root. Roots are dense and hold their ground.
Lamb’s quarters (Chenopodium album)
Canada thistle (Cirsium arvense)
A perennial from France, it has ovate, stalkless, hairless cotyledons and an underground root system that may be more than 30 centimetres below the surface. Older, well-established plants could have as many as 200 nodes for sending up new plants, so every single piece of root must be removed. Seeds can last for 21 years in the soil, but must be near the surface to germinate. One plant can produce an astounding 40,000 seeds or so each year.
Field bindweed (Convolvulus arvensis)
A perennial from Europe, it has long-stalked, heart- or kidney-shaped cotyledons. Seeds can live up to 50 years in the soil, and roots can be as deep as nine metres and travel 30 metres; remove all root pieces with a garden fork.
Pineapple weed (Matricaria matricarioides)
Yellow nut sedge (Cyperus esculentus)
A perennial native to North America, it has cotyledons that resemble grass and an easily identifiable triangular stem. New plants are formed from underground tubers. Remove the entire plant while young to avoid leaving the tubers behind—in one growing season, a single tuber can produce almost 2,000 new plants and spread two metres. This plant can only overwinter as a tuber in the upper 15 centimetres of soil if temperatures are above –7C.
Dandelion (Taraxacum officinale)
Purslane (Portulaca oleracea)
Poison ivy (Rhus radicans)
A perennial native to North America, it has cotyledons with a single, vertical stem. Use non-selective herbicides to control. Small plants can be carefully dug out with a trowel, but wear gloves and wash everything with soap and water afterwards—85 per cent of the population is allergic to its sap.
Common groundsel (Senecio vulgaris)
Common plantain (Plantago major)
An annual, biennial or perennial from Europe that has invaded gardens almost to the Arctic Circle, it has spatula-shaped cotyledons with three faint veins. Use a broadleaf herbicide in lawns or dig out with a trowel.
Cited from: http://www.canadiangardening.com/plants/native-plants-and-wildflowers/is-it-a-weed-or-not/a/1447/3