There are a wide range of strawberry cultivars including those that are well adapted to winter cold and spring frost or summer heat and drought. Cultivars also vary widely in their need for chilling and time of bloom and ripening period. Most production is focused in temperate and Mediterranean climates, between latitudes 28 and 60°, with average mid-summer temperatures of 15-30°C.
Strawberries require a steady supply of water. This is most critical at establishment and during fruit development (from petal fall to end of fruiting), otherwise there will be a reduction of leaf area, photosynthetic rate and yield. Creating water stress – in specific 60 day plants - at the later stage of flowering, through reducing crop irrigation, can be used, however, to increase flower numbers and fruiting. In contrast, too much water leads to malformed fruit.
Very high temperatures can have a negative effect on growth and are detrimental to photosynthesis and productivity. Temperatures above 25°C can reduce fruit set, levels of Total Soluble Solids (TSS) and at the same time, hasten fruit development. High temperature can also reduce fruit size, and lead to tissue damage, softness and breakdown near the berry surface.
Pollination is largely carried out by insects such as bees and bumble bees – which are increasingly introduced to crops grown in tunnels to aid this process. Poor pollination – particularly in cold conditions or due to lack of boron, or damage due to high or low temperatures, insects or disease - can result in malformed fruit. This poor, distorted berry formation is due to the restricted formation of auxins. However, while auxins can be applied to the crop to counteract these effects, this can have a negative effect on the fruit’s ripening process.
Strawberry is an herbaceous perennial whose upper parts die back at the end of the season. It is, however, for commercial reasons, often cultivated as an annual crop. The central stem or crown of the plant supports the leaves, roots, stolons and inflorescences - Figure 1 - and typical growth stages are illustrated in Figure 2. Plant development is regulated by environmental factors – notably photoperiod and temperature.
Roots are largely formed in the spring and autumn when temperatures are cool. They emerge from the base of the crown and between 50-90% of all roots are found in the top 10-15cm of soil. Water and nutrients are taken up by feeder roots, which last only for a few days or weeks, and are constantly replaced. Primary roots are used for the transport of water and nutrients to the crown, and commonly last for more than one season. Root biomass declines during fruiting but increases again in the autumn prior to the onset of winter temperatures.
Leaves are produced during the whole season, mostly under long-day conditions. However, leaf production ceases when temperatures drop below 0oC or exceed 30oC. At the base of each leaf is an auxiliary bud which, depending upon conditions, either produces lateral crowns with inflorescences or stolons (runners). Runners are produced from base buds, whereas inflorescences are produced from terminal buds of the lateral crowns. Flowers are produced when conditions are not favourable for the production of runners. The first fruit on the runner will mature first, and as more fruit develop they mature in turn.
Stolons support daughter plants which can also produce plants in their own right. These daughter plants are usually self-sufficient and can survive on their own after 2-3 weeks. Most commercial strawberries are either short-day or day-neutral plants (Figure 3).
Strawberry healthy root systemsShort-day plants produce flowers buds either under short-day conditions (less than 14 hours of day length) or when temperatures are less than 15°C. In these short-day types, stolons are produced after flowering, normally when temperatures are between 20-30oC, or when days are longer than at least 10 hours.
In climates with cold winters, flower buds of short day types are normally formed in the late summer and autumn. Short-day cultivars can be grown in mild, sub-tropical climates, but flower bud formation will be restricted by hot temperatures.
Day-neutral plants produce crowns and flower buds around three months after planting, regardless of day length throughout the summer and in due course, produce stolons. Again, high day/night temperatures of 30/26oC will inhibit flower formation.
Shorter days and lower temperatures induce plant dormancy. Chilling at -1 to -10oC is required in most cultivars to break this dormancy – those varieties bred for warmer climates have a greatly reduced chilling need.
In sub-tropical growing conditions, production is better from planting material produced in cool climates compared to that created locally. Once fully hardened, the crowns of the hardiest cultivars can survive temperatures as low as -45oC. While strawberries can tolerate low winter temperatures, frost during flower development damages blooms. In some countries, irrigation is used to provide frost protection. In this situation, overhead irrigation water applied prior to frosts, freezes around the leaf, releasing heat which protects the plant from damage. Strawberries grown under plastic film can tolerate frosts down to -50oC.
Tolerance to cold temperatures varies with the growth stage of strawberries. Flowers are the least tolerant to cold. The temperatures shown are where commercial loss of production occurs due to frost (table 4).