Horticulture/Fragaria x ananassa

Garden Strawberry (Fragaria x ananassa and related cultivars) is the most common variety of strawberry cultivated worldwide. Like all strawberries, it is in the family Rosaceae; its fruit is more technically known as an accessory fruit, in that the fleshy part is derived not from the plant's ovaries (achenes) but from the peg at the bottom of the bowl-shaped hypanthium that holds the ovaries.

The Garden Strawberry originated in Europe in the early 18th century, and represents the accidental cross of Fragaria virginiana from eastern North America, which was noted for its fine flavor, and Fragaria chiloensis from Chile and noted for its large size.

Cultivars of Fragaria x ananassa have replaced in commercial production the Woodland Strawberry, which was the first strawberry species cultivated in the early 17th century.

Description
Strawberry plants are rosette-forming perennials which spread by thin runners (stolons). The leaves are trifoliate, dark green but paler and haired on the undersides, on long petioles emerging from the crowns. The flowers have yellow centers, with the 5 petals white or occasionally pink, borne underneath the foliage canopy. The fruits need no describing.

Strawberry varieties vary remarkably in fruit size, colour, flavour and shape, the degree of fertility, season of ripening, liability to disease and constitution of plant. Some vary in foliage, and some vary materially in the relative development of their sexual organs. In most cases the flowers appear hermaphroditic in structure, but function as either male or female.

Cultivation


Strawberries are most often grown in raised beds or containers, though they can also be grown in the field.

The bulk of modern commercial production uses the plasticulture system. In this method, raised beds are formed each year and covered with black plastic, which prevents weed growth and erosion, under which is run irrigation tubing. Plants, usually obtained from northern nurseries, are planted through holes punched in this covering. Runners are removed from the plants as they appear, to encourage the plants to put most of their energy into fruit development. At the end of the harvest season, the plastic is removed and the plants are plowed into the ground. Because strawberry plants more than a year or two old begin to decline in productivity and fruit quality, this system of replacing the plants each year allows for improved yields and more dense plantings. However, because it requires a longer growing season to allow for establishment of the plants each year, and because of the increased costs in terms of forming and covering the mounds and purchase plants each year, it is not always practical in all areas.

The other major method is to maintain the same plants from year to year. The runners of established plants should be allowed to root in the soil adjoining the plants, which should, therefore, be kept light and fine, or layered into small pots as for forcing. As soon as a few leaves are produced on each the secondary runners should be stopped. When the plants have become well-rooted they should at once be planted out. They do best in a rather strong loam, and should be kept tolerably moist. The ground should be trenched 50–100 cm deep, and supplied with plenty of manure, a good proportion of which should lie just below the roots, 25–30 cm from the surface. The plants may be put in on an average about 50–60 cm apart.



Varieties
Strawberries are often grouped according to their flowering habit. Traditionally, this has consisted of a division between "June-bearing" strawberries, which bear their fruit in the early summer and "Ever-bearing" strawberries, which often bear several crops of fruit throughout the season. More recently, research has shown that strawberries actually occur in three basic flowering habits: short day, long day, and day neutral. These refer to the day length sensitivity of the plant and the type of photoperiod which induces flower formation. Day neutral cultivars produce flowers regardless of the photoperiod. Most commercial strawberries are either short day or day neutral.

Uses
Strawberries are often used to flavor other foods, such as ice cream. Apart from its interest as a dessert fruit, the strawberry draws interest due to the peculiarities of its structure, its tendency towards variation, and the gardener's success in exploiting this tendency.

Maintenance
Most cultivars are somewhat self fertile, but good bee activity has been shown to improve pollination, which results in larger and better shaped berries. Commercial growers usually place beehives within range of the fields to increase bee populations.

The plantation should be renewed every second or third year, or less frequently if kept free of runners, if the old leaves are cut away after the fruit has been gathered, and if a good top-dressing of rotten dung or leaf mold is applied. A top-dressing of loam is beneficial if applied before the plants begin to grow in spring, but after that period they should not be disturbed during the summer either at root or at top. If the plants produce a large number of flower-scapes, each should, if fine large fruit is desired, have them reduced to about four of the strongest. The lowest blossoms on the scape will be found to produce the largest, earliest and best fruits. The fruit should not be gathered till it is quite ripe, and then, if possible, it should be quite dry, but not heated by the sun. Those intended for preserving are best taken without the stalk and the calyx.

A mulching of straw put between the rows in spring serves to keep the ground moist and the fruit clean, as well as to afford nourishment to the plants. Unless required, the runners are cut off early, in order to promote the swelling of the fruit. The plants are watered during dry weather after the fruit is set, and occasionally till it begins to colour. As soon as the fruit season is over, the runners are again removed, and the ground hoed and raked.

As the plants begin to set fruit, mulch with straw to prevent the fruits from touching the bare soil and to prevent soil being splashed onto the fruit during rain or irrigation.

Strawberries should be cut back to the crown following fruiting, especially if the leaves show signs of disease. They also benefit from being divided and having composts added to the soil.

Runners should be removed if they grow out of the row. These can be used to start new plants in a different row if desired.

Because the foliage of a strawberry patch will retain moisture at the soil surface, care must be taken not to overwater once fruit begins to ripen. An ill-timed rain can result in mold, rot, and slug damage.

Propagation
For purposes of both commercial and garden production, the plants are propagated from runners and generally distributed as either bare root plants or plugs. Cultivation follows one of two models, annual plasticulture or a perennial system of matted rows or mounds. A small amount of strawberries are also produced in greenhouses during the off season.

While rarely if ever done commercially, strawberries may also be propagated by seed, and a few seed propagated cultivars have been developed for home use. Seeds are acquired commercially or saved from fruit ripened early in the summer. They may at once be sown, either in a sheltered border outdoors or in pots, or better in March under glass, when they will produce fruits in June of the same year. The soil should be rich and light, and the seeds very slightly covered by sifting over them some leaf-mould or old decomposed cow dung. When the plants appear and have made five or six leaves, they are transplanted to where they are to remain for bearing. The seeds sown in pots may be helped on by gentle heat, and when the plants are large enough they are pricked out in fine rich soil, and in June transferred to the open ground for bearing.

Forcing for early transplant
The runners propagated for forcing are layered into 75 mm pots, filled with rich soil, and held firm by a piece of raffia, a peg or stone. If kept duly watered they will soon form independent plants. The earlier they are secured the better.

When firmly rooted they are removed and transferred into well-drained 150 mm pots, of strong well-enriched loam, the soil being rammed firmly into the pots, which are to be set in an open airy place. In severe frosts they should be covered with dry litter or bracken, but do not necessarily require to be placed under glass. They are moved into the forcing houses as required.

The main points to be kept in view in forcing strawberries are:
 * use strong stocky plants, the leaves of which have grown sturdily from being well exposed to light, and
 * grow them slowly until the fruit is set.

When they are first introduced into heat, the temperature should not exceed 8 °C to 10 °C, and air must be freely admitted; should the leaves appear to grow up thin and delicate, less fire heat and more air must be given, but an average temperature of 13 °C by day may be allowed and continued while the plants are in flower.

When the fruit is set the heat may be gradually increased, till at the ripening period it stands at 18 °C to 24 °C by sun heat. While the fruit is swelling the plants should never be allowed to get dry, but when it begins to colour no more water should be given than is absolutely requisite to keep the leaves from flagging. The plants should be removed from the house as soon as the crop is gathered. The forced plants properly hardened make first-rate outdoor plantations, and if put out early in summer, in good ground, will often produce a useful autumnal crop.

Harvest
Strawberries are ripe when fully red. They should be picked often, as the fruit quickly rots after ripening, and is highly susceptible to gray mold which can spread to other fruits or to the foliage.

After picking, strawberries should be either consumed quickly or frozen, as they are quite prone to molds once picked. If freezing, remove the calyx and stem and freeze fresh.

Fungal diseases
The most troublesome fungal pests are mildew and leaf spot. The former, like all mildews, attacks the leaves and spreads to the fruit, these being covered with the white mycelium. The fungus is identical with that causing mildew in hops (Sphaerotheca humuli), and its development is greatly furthered by exposure of its host to cold draughts or low night temperatures. Spraying the foliage with potassium sulfide (K2S) (mixed with water at a 1:40 ratio by volume) should hold it in check, but the plants should not be sprayed when the fruit is developing. The "leafspot" is caused by the fungus Sphaerella fragariae. The first symptom of this attack is the appearance of small, circular, white spots on the leaves, having a broad, definite, dark reddish margin.

On these spots a whitish mould develops, and this is followed later by the perfect form of the fungus, the fruits of which appear to the naked eye as small black spots seated on the white dead spot on the leaf. Potassium sulfide may be used as for the mildew, or, perhaps better, Bordeaux mixture. Some recommend cutting off the leaves after fruiting and turn the beds over so as to destroy the fungus in the leaves.

Arthropod pests
The grubs of the cockchafer (Meloloniha vulgaris) and the Rose chafer (Cetonia aurata) frequently feed upon the roots of the strawberry and do considerable damage, while the larvae of the Ghost Moth (Hepialus humuli) and garden swift moth behave in a similar way. The imago of Cetonia aurala also frequently damages the flowers of the strawberry by devouring their centres, and is often troublesome in this way in forcing-houses particularly. The carnivorous ground beetles, particularly Pterostichus nigra and Harpalus rufimanus, when the fruit is ripe attack it at night, returning to the soil in the daytime. They are to be caught by placing jars containing some attractive matter, such as meat and water, at intervals about the beds with their mouths sunk level with the surface of the soil. Millipedes also are often found in the ripe fruit, but occur mostly where the soil is very rich in organic matter and poor in lime.

Slugs and snails
Slugs and snails also snack on the fruit, as do birds. Slug pellets can be used to reduce their numbers, with child and animal safe versions available. Organic solutions to slug attacks include beer baiting.

Higher animal pests

 * Birds
 * Mice, rats, voles, and squirrels
 * White-tailed Deer
 * Rabbits

References and external links

 * F. ananassa data from GRIN Taxonomy Database
 * Growing Strawberries - University of Illinois