Cornflowers are extremely colourful hardy annuals. They look great in beds and borders, especially when part of an annual bedding display or a cottage garden, flowering from late spring and summer into autumn.
I. Appearance and Characteristics
Centaurea cyanus, commonly known as cornflower or bachelor’s button, is an annual flowering plant in the family Asteraceae native to Europe. In the past, it often grew as a weed in cornfields (in the broad sense of “corn”, referring to grains, such as wheat, barley, rye, or oats), hence its name. It is now endangered in its native habitat by agricultural intensification, particularly by over-use of herbicides. However, Centaurea cyanus is now also naturalised in many other parts of the world, including North America and parts of Australia through introduction as an ornamental plant in gardens and as a seed contaminant in crop seeds.
Centaurea cyanus is an annual plant growing to 40–90 cm tall, with grey-green branched stems. The leaves are lanceolate and 1–4 cm long. The flowers are most commonly an intense blue colour and arranged in flowerheads (capitula) of 1.5–3 cm diameter, with a ring of a few large, spreading ray florets surrounding a central cluster of disc florets. The blue pigment is protocyanin, which in roses is red. Fruits are approx. 3.5 mm long with 2–3 mm long pappus bristles. It flowers all summer.
II. How to Grow and Care
Sunlight
Cornflower requires long periods of sunlight for proper development. Although, it should not be under direct sunlight during the hot summer days, when the sunlight is very intense, as the plant might start wilting and developing burns. It is best to keep it in a partial shade.
Sunlight requirements for cornflower sometimes cannot be met during winter, when the days are shorter. Exposing the plants to artificial light during the night will effectively extend light exposure and provide them with enough light to get through the winter. This practice also encourages early flowering.
Temperature and Humidity
When it comes to temperature, cornflower tolerates both a light freeze as well as a hot summer day. This plant thrives in 60 F to 80 F temperatures, but may need 85 F to 95 F to reach flowering maturity. Cornflower grows best with an average humidity range of 30 to 50 percent. Keep a close eye on your garden during humid spells, though, as this plant is susceptible to fungal disease under these conditions.
Watering
Cornflower needs moderate amounts of water for proper growth, so the watering should be done once every 3-4 days during the spring and fall. When temperature in summer is high, large amounts of water evaporate, so watering should be done twice a day – in the morning and in the evening. However, to avoid accumulation of water in the soil, the plant can also be sprayed with water. During the winter, cornflower should be watered once in every 1-2 weeks, as it is better to keep the soil slightly dry.
Soil
Cornflower is adapted to various soil types. It can grow in both heavy clay and rapid draining sand. However, for the most favorable growing conditions, it is best to use fertile, loose and well-drained sandy soils. A mix of garden soil, a bit of sand and wood ash is the best potting substrate.
In order to prevent pests and diseases from occurring (or recurring), cornflower should not be planted on the same plot for many consecutive years; when it comes to potted plants, substrate should be replaced annually.
Fertilizing
Cornflower is tolerant to infertile soils, but adequate nutrition will promote blossom. Before sowing, generally an appropriate base fertilizer is applied (preferably an organic or mineral fertilizer) during soil preparation. Nitrogen-based fertilizers can be applied during vegetative phase to promote growth.
Half a month before bloom time, fertilizers based on phosphorus and potassium (e.g. monopotassium phosphate) can be applied to promote flowering. Try to avoid excessive fertilization, because otherwise, it will only benefit the growth of weeds, resulting in an overgrowth of plants but reduce flowering.
Planting Instructions
The Root system of cornflower has one main root, making it suitable for direct sowing. Sowing should be performed in spring and fall. Sowing in spring needs to be carried out as early as possible. The suitable temperature for germination is 15 to 20 ℃, and the seedling will emerge after 4-7 days.
Before sowing, mix the seeds and fine sand in a ratio of 1:2-3, and sow them evenly. Comb the plot gently with a rake afterwards. After sowing, the covering soil should not be too thick, as it is advised to cover the seeds tightly, otherwise the seedlings might grow up unevenly.
Crush the soil with a tool to combine the seeds with the soil tightly. Finally, cover it with a non-woven fabric or a straw curtain to increase ground temperature and promote seed germination. It is best to choose a windless day for sowing. The sowing height should be reduced if the wind is strong.
Pruning
Cornflower grows side branches naturally, and too many of them will cause the flowers to grow smaller. Producing larger flowers can be achieved by cutting off some terminal buds. During the mid-summer, climate conditions are usually unfavorable for growth of cornflower, and the leaves start to wither and turn light green or yellow. At this point, the plant can be cut off to a third or to half of its height. When the weather becomes cool, it will rejuvenate and grow new leaves and flowers.
During winter it is advised to cut off the roots of cornflower that have perennial roots below the ground, to ensure that the plant survives cold temperatures and frosts.
Propagation
Cornflower is mainly propagated by sowing and division. Its seeds are scattered easily, so it’s best to harvest them in batches, when the inflorescences turn yellowish-brown after the petals have fallen. The seeds should be sown in spring or fall.
If the sowing is performed in fall, it needs to be at a place protected from low temperatures, or planted indoors in a seedling tray and then transplanted in spring. It has strong viability and can self-propagate. The seeds successfully germinate on their own with enough water.
Perennial species can also be propagated by division. Division includes digging out a cluster of plants 2-3 years old, and separating them with a sharp knife, each cluster preferably containing 3-5 small buds. Individual clusters serve as new planting material. Division is generally carried out during spring or fall. The soil must be kept moist before the new roots have properly grown.
Potting and Repotting
When growing cornflower in pots or containers, make sure to keep the soil on the dry side to mimic pasture conditions. Porous clay or terracotta pots are best. Choose well-drained soil with perlite, or use a soilless medium made from organic material (similar to a cactus potting mix). Deadhead your potted cornflower for a tidy appearance, and be prepared for a short flowering season when grown this way.
Pests and Diseases
Common Pests & Plant Diseases
An annual with a short growing season, cornflower rarely suffers from serious insect or disease problems. Still, when kept in wet, humid conditions, powdery mildew can occur late in the season, appearing as white spots on the leaves. This fungus rarely kills the plant and treatment is usually not necessary since the plant’s life cycle is short-lived anyway.
Aphids and mealybugs rarely, but sometimes, appear on a crop of cornflower. Control them by reducing the population with bursts of water from the garden hose. Avoid spraying pesticides around cornflowers, even organic ones, as both are harmful to bees and other pollinators that feed on them.
Common Problems with Cornflower
Although rare, cornflower can fall victim to fungal infections when overwatered and crowded. Make sure to adhere to watering recommendations and thin seedlings to allow for proper airflow around the mature plant. In drought conditions, cornflower is prone to wilting, which can cause your crop to fall over and look unsightly. Usually, a good drink is all that is needed to perk them up.
III. Uses and Benefits
- Food
The flowers of Centaurea cyanus can be eaten raw, dried or cooked. Dried petals are used in foods, including in spices. Their main purpose is to add colour to food. There are cheeses or oils that contain raw petals. Petals can also be added to salads, drinks, or desserts for garnishing purposes in raw or dried form.
- Beverages
Dried petals are also used in teas and other beverages. Blue cornflower petals are sometimes one of the ingredients in Lady Grey tea.
Ornamental use
Centaurea cyanus is used as an ornamental plant. There are varieties with blue, white, purple, pink or even black petals.
- Pigment
The blue color of Centaurea cyanus is due to protocyanin, an anthocyanin pigment that is also found in roses. Different anthocyanins derived from Centaurea cyanus are used as natural additives in food products, such as yoghurts.
- Medicinal purpose
Centaurea cyanus contains a wide range of pharmacologically active compounds, such as flavonoids, anthocyanins and aromatic acids. The flower head finds application in herbal medicine, but leaves and seeds are also used for pharmacological purposes, albeit to a lesser extent.
In particular, extracts from the flower heads have anti-inflammatory properties used in the treatment of minor ocular inflammations. Antioxidant properties are high due to ascorbic acid and phenolic compounds. Furthermore, extracts of the flower head and vegetative parts of the plant were shown to have gastroprotective effects due to their content of quercetin, apigenin and caffeic acid derivatives.
- Phytoremediation
Centaurea cyanus has been evaluated for phytoremediation of soils contaminated with lead. Inoculation of the contaminated soil with Glomus spp. (fungus) and Pseudomonas spp. (bacterium) would significantly enhance the biomass production and lead uptake of Centaurea cyanus.
Find Where to Buy the Best Cornflower (Centaurea cyanus)
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