Some weeds are annoying - some are a real problem! Here are some of the most invasive weeds.
Get familiar with these plants by doing a Google images search.
Japanese Knotweed (Polygonum cuspidatum)
Introduced by the Victorians as ground cover, this invasive weed can be incredibly distuctive. Japanese Knotweed can grow from a very small root making it very difficult to irradicate completely. Systematic weed killers, such as any brand containing glyphosate, are best as these attack the root through the leaves. You will need to apply the weedkiller repeatedly until it is completely gone but don't wait until the weed gets big before treating it.
Landowners are not legally obliged to remove Japanese Knotweed, unless it is causing a nuisance to neighbouring property. However it is an offence to plant, or cause Japanese Knotweed to grow, in the wild.
All parts of the plant and any soil contaminated with it are classed as controlled waste, so you need to dispose of it carefully to make sure it does not spread. If you are using a contractor to remove the waste for you, they must be registered with the Environment Agency as a waste carrier.
Russian Vine (Fallopia baldschuanica)
You may find this plant in your local garden centre. It is often planted when someone wants to hide something ugly very quickly - but at what cost? A relative of Japanese Knotweed, Russian Vine can grow 40ft in a year and actually has some attractive flowers that bloom in late summer. Stopping it from growing is the problem and it will likely be considered a 'weed' by a neighbour rather than the person who planted it. The best way to kill it is often by cutting it back to the ground and apply a stump killer.
Hemlock (Conium maculatum)
The parsley-like leaves are the give-away, but one to be aware of and destroy with weedkiller. It is a herbaceous biennial plant which grows between 1.5–2.5 metres (5–8 ft) tall, with a smooth green stem. The leaves are finely divided and lacy, overall triangular in shape, up to 50 centimetres (20 in) long and 40 centimetres (16 in) broad. The flowers are small, white and clustered upto 10–15 centimetres (4–6 in) across. When crushed, the leaves and root emit a rank, unpleasant smell.
Giant Hogweed (Heracleum mantegazzianum)
This plant can grow to 3.5m (11ft) and looks quite similar to Hemlock. It's sap is an irritant and can make the skin hypersensitive and blister.
Indian Balsam (Impatiens glandulifera)
Indian Balsam belongs to the Busy Lizzy family so it is not surprising that it has a pretty flower. However, exploding seed heads mean that this plant can travel far and wide and become a problem for you and your neighbours.
Greater Duckweed (Lemma polyrrhiza)
This is an oxygenating plant that will quickly cover the surface of your pond. It is often attached to other plants that you may buy for your pond. You can clear this by hand - obviously, it would not be good to use weedkiller on a pond! When cleared regularly, it can shade the pond from the sun (reducing algae so the water stays clear) and give fish some shelter from predators.