Despite the negative agricultural and environmental impacts of the more aggressive
Centaurea species, there are many ways in which they benefit humans as well. For instance, due to their moderate to high nectar production, which can occur over a comparatively long duration, many species of
Centaurea are popular food sources for
insects that may otherwise attack certain
crops. It may be advisable for some types of farms to allow certain Centaurea species, such as
cornflower (
C. cyanus) in a European setting, to grow adjacent to fields. These areas are known as
beetle banks, though they support and attract a diversity of beneficial life beyond beetles. When certain Centaurea species are present, some pests may be drawn away from crops, and predatory insects and
arachnids that feed upon pest insects will be better-supported by these more naturalized areas. They additionally have the beneficial aspect of supporting
pollinators, unlike many field crops such as
maize. Moreover, being untreated with
pesticides and providing more
diversity, plants growing in more wild areas adjacent to farms produce more insects that attract and support birds which can also feed on pests that would harm crops. Insect production is especially high for beetle banks that have enough plants that serve in the role of host plant for immature insects, rather than just in the roles of adult food and/or shelter provision. Some plants which are considered invasive or problematic in certain areas can have beneficial qualities that outweigh their negative qualities from a human and/or human agricultural point of view, although this sometimes requires some human management – particularly if adequate biological control has not been established for the more aggressive species. An example is wild parsnip,
Pastinaca sativa, which produces florets that feed predatory (and other beneficial) insects as well as large tubular stems that provide winter shelter for native bees, wasps, and other organisms that can be beneficial for agriculture. The plant is considered invasive in some areas of the United States and is also often considered undesirable due to its ability to cause contact skin irritation. However, it also serves as a host plant for the black swallowtail butterfly, helps to bring nutrients up from soils with its deep taproot, and possesses evergreen foliage even in climate zones such as US zone 6. This foliage increases soil warmth and moisture which can be beneficial for certain types of life. Perhaps the most dramatic example of a generally disliked plant's beneficial qualities being usually overlooked is the often-despised ragwort,
Jacobaea vulgaris, which topped the list by a large amount for nectar production in a UK study, with a production per floral unit of (2921 ± 448μg). This very high nectar production, coupled with its early blooming period, makes the plant helpful for the establishment of bee colonies in spring — a period that is often not well-served by commercial flower meadow seed mixes. It also has the situationally-beneficial quality of being a spring
ephemeral, as well as an annual that lacks difficult-to-combat roots. Plants that provide necessary structural supports for
invertebrate and small
vertebrate predators can help to keep overall pest populations low. (
C. solstitialis), an
invasive weed that yields a fine
honey The abundant
nectar produced by
C. solstitialis flowers attracts many
pollinators. This is another reason for the success of the (situationally) highly
invasive species. Due to genetic differences related to evolutionary adaption, not all members of
Centaurea produce the same amount of nectar. Growing conditions, such as climate and soil, can have a very strong impact, even if the plants grow and flower. For instance, cornflower plants,
Centaurea cyanus, produced 33% less seasonal nectar than
Centaurea nigra in a UK study. Spotted knapweed as well as other species are rich in
cnicin, a
bitter compound found mainly in the leaves and often used to flavor the
digestif amaro. In western
Crete,
Greece a local
variety of
C. calcitrapa called ''
( "little pig") also has its leaves eaten boiled by the locals. In the same island an endemic local species, C. idaea'' called '
(), ' () or '''' (), has its leaves eaten boiled by the locals too. Some species are cultivated as
ornamental plants in
gardens. As regards other aspects of
popular culture,
cornflower (
C. cyanus) is the
floral emblem of
Östergötland province (Sweden) – where is it called
blåklint, literally "blue mountain" – and of
Päijät-Häme region in
Finland, where it is known as
ruiskaunokki ("
rye-beaks") or
ruiskukka ("rye-flower"). It is also the national flower of
Estonia where its local name
rukkilill means "rye-
lily",
Belarus where it is called
vałoška (), and one of those of Germany where it is called
Kornblume ("cornflower"). The origin of the name "
caltrop" for the ancient low-tech
area denial weapon is probably in some way connected with
C. calcitrapa and its spiny seeds. This plant is attested to by the colloquial name "caltrop" at a time when the weapons were still called by their
Roman name
tribulus. Lastly, the
color cornflower blue is named after
C. cyanus. Cornflower is also used as a cut flower. ==Systematics and taxonomy==