Plant collectors have been searching the globe for new varieties of ornamental plants to satisfy the garden trade. This collecting has been responsible for the importation of several thousand species of which around nine hundred have become established in the wild. Some of these species have a naturally vigorous growth strategy. Japanese Knotweed (Fallopia japonica) and Himalayan balsam (Impatiens glandulifera), for example exhibit extreme competitiveness and are now common sights on riverbanks and in neglected areas of Britain. Other more recent introductions such as Australian Swamp Stonecrop (Crassula helmsii), Parrots Feather (Myriophyllum aquaticum) and Floating Pennywort (Hydrocotyle ranunculoides) are now present in the British countryside and exhibit distinctly invasive growth strategies. At present Crassula helmsii is by far the most widely distributed of these three species in Britain and as such has been the focus of much research regarding the plants impact and subsequent control.

Plants have undoubtedly played an important role throughout human history. Many examples of these important plants are known to people throughout the world and include tobacco, cotton, and sugar and their importance in shaping several centuries of American history. Spices and their importance in trade throughout Europe and Asia, and cereal crops and the rise of agricultural societies in the Middle East.

Effective weed control is critical for maximising crop yields, reducing the weed seed bank and meeting quality standards at harvest. Crop weeds can most effectively be managed by integrating chemical and non-chemical control. Weed control should be planned well before planting by deciding:

Which weeds are likely to be a problem?
Which crop to plant? Furthermore, which crop will be planted the following season?
What planting rate and spacing is needed?
Whether herbicides will be needed?
Whether to use pre-emergent or post-emergent herbicides?
What range of herbicides are available for use in a particular crop?
Do herbicide groups need to be rotated to avoid herbicide resistance?

Crop selection is important in managing weed levels in crops and avoiding unnecessary herbicide applications.

Continuous winter crops may result in severe weed problems. Rotation into summer crops such as sorghum may then be beneficial. At least three years of summer cropping is usually needed for lasting reductions in winter weed populations.
Grain legumes and broadleaf crops such as sunflowers should be avoided on country where broadleaf weeds are a problem. The broadleaf herbicide options for these crops are limited and expensive for legumes and almost non-existent for sunflowers and sesame.
Aggressive 'early growers', or crops with a range of effective and affordable herbicides are preferred for 'dirty' country. For example, barley is an ideal winter crop where weeds are a problem because it is very competitive and enables a range of herbicides to be used.

Weed identification and size - Weeds must be correctly identified before the best suited herbicide can be selected. Weeds should be sprayed while they are small as large weeds are harder and more expensive to kill. In some situations, allowing the weeds such as wild oats to continue growing to the large weed stage will result in a reduction in yield. This will also maintain the weed seed bank if the weeds are allowed to seed. Ideally, grass weeds should be sprayed while they are very small. In summer, this is usually within 14 days of germinating rains. Broadleaf weeds should be sprayed while they have no more than four true leaves. Young weeds are less likely to be moisture stressed. Higher herbicide rates are usually needed on advanced weeds and control may be less reliable, particularly in hot, dry conditions. In fallow situations, it is recommended that maximum herbicide rates be applied to grass weeds early in the summer. Lower rates may result in partial control, persistence of the weeds and reduced effectiveness of follow-up sprays.