Strawberries in your garden
The strawberry is a member of the rose family and belongs to the genus Fragaria derived from the Latin, fragare, meaning pleasant aroma. Strawberries originated from six main species, mostly from Europe, North America and Chile. The first record of the strawberry was an illustration that appeared in Mainz Herbarius, published in 1485. The first strawberries were collected from wild types and plants obtained from their natural habitat and cultivated in gardens. Tusser in “Five Hundred Pointes of Good Husbandrie” wrote in 1557:
“Wife into a garden and set me a plot
Most strawberries grown up to the late 1700s were selections of individual species and the fruit was smaller than a five cent coin. Since then, science has changed the strawberry dramatically. Cross breeding between the various species has meant that strawberries have increased dramatically in size and quality. Most modern strawberries are categorised as Fragaria X ananassa, this name reflecting the fact that modern strawberries are a mix of multiple species.
Strawberries have been grown in New Zealand since the arrival of early European settlers. Amongst the varieties grown in New Zealand since the early days are varieties such as Captain Cook, which can still be found in collections and which has been used in breeding some of the types now available in New Zealand. Redgauntlet has been a popular variety in New Zealand. This variety, bred by Scottish scientists and named after a character in a Walter Scott novel, was first introduced into New Zealand in 1965. Although Redgauntlet is still available, the variety has been superseded by more recent introductions with better sized fruit and better flavour. Varieties presently available for the home garden, mostly bred at the University of California, include Chandler, Pajaro, Aptos, Yolo and Seascape. More recently, varieties bred in New Zealand by HortResearch scientist Geoff Langford, such as Gabrielle or Sundae™, have become available. Modern varieties are a huge improvement in terms of yield, pest and disease resistance, size, keeping quality and, generally, flavour. They have been bred to produce consistently large, attractive fruit. The season has also been extended with modern varieties and some varieties now fruit from spring to late autumn.
Strawberries are the most popular berry grown in the world. They are cultivated in most places in the world and world production is estimated at 2.8 million tonnes. USA , Spain and Japan are the biggest producers worldwide with New Zealand producing just 6000 tonnes.
Strawberries are primarily eaten for their taste, but they do confer positive health benefits. In medieval times, strawberries were regarded as an aphrodisiac and soup made of strawberries, borage and soured cream was traditionally served to newly-weds at their wedding breakfast. History records strawberries being used for, amongst other things, fastening loose teeth and treating mouth ulcers. A tea made from the leaves was used as an anti-diarrhoeal agent by American Indians.
Researchers are continuing to find new health benefits from eating this wonderful fruit. Recent work by Dr John Maas and others in the USA has shown that strawberries contain very high levels of a compound called ellagic acid, especially in the seeds and leaves, but the compound is also present in useful amounts in the fruit. Ellagic acid has a wide range of biological activity including anti-carcinogenic and anti-mutagenic properties. The plant has developed this compound to act as part of its natural defence mechanism as it has both anti-fungal and anti-insect feeding activity.
Strawberries are also high in Vitamin C, Vitamin K and fibre and contain useful amounts of Vitamins B1, B5 and B6, and potassium, folate, manganese, magnesium, iodine, biotin (a B group vitamin) and tryptophan (an essential amino acid for human nutrition).
Red-fleshed strawberries are also moderately high in anthocyanins. Anthocyanins are powerful antioxidants that are beneficial in protecting against a range of cancers and heart disease. As an example, a study published in the November 2003 issue of the Journal of Agriculture and Food Chemistry analysed eight strawberry cultivars for phenols, flavonoids and anthocyanins, and antioxidant capacity. All cultivars were able to significantly inhibit the proliferation of human liver cells.
Dr William Butler famously said “Doubtless God could have made a better berry, but doubtless God never did”.
The only downside for strawberries is that they can cause allergies in some people and that they do contain small quantities of oxalate. When oxalates become concentrated in body fluids, they can crystalise and lead to kidney and gall stone problems.
Strawberries can be grown in many ways in the garden. Commercially, they are grown in rows through black polythene and are usually spaced about 12 cm apart depending on variety. Research by Dr Greg Pringle of UNITEC has shown that the more space given to each plant, the greater the production, but closer spacing generally means more production for the same area. If space is not limited, wider spacing is better as this helps with air movement around the plants and subsequently with disease control.
In the home garden, they can be grown in containers, in hydroponic systems, or like the commercial growers do, in soil. Do not grow strawberries following solanaceous crops such as potatoes or tomatoes as these crops encourage the build-up of root diseases such as Verticillium sp. to which strawberries are very sensitive.
Fruit coming into contact with soil will rot, so keeping fruit away from the soil in some way will help. Straw is commonly used but any material such as plastic, newspaper or clean bark can be used.
Strawberries are rich in nutrients and these originate from elements in the soil. Feeding the soil with the nutrients removed by the plants while growing and fruiting is a key component in sustainable production systems. Soil should be slightly acid with a pH of around 6, although with good soil structure, strawberries are very tolerant of lower acidity soils. A strawberry plant producing about 300 g of fruit will remove about 0.6 g of nitrogen, 0.6 g of potassium and 0.1 g of phosphorous from the soil. Using a typical compound fertiliser (12N 5P 14K) will mean that 5 g per plant is required. Over fertilising can result in poor growth and production, as plants are very sensitive to high nutrient levels. The same amount of nutrients can be provided from composts, although it can be difficult to calculate the nutrients provided. See the information sheet on soil quality, as although strawberries can be grown on soils with poor structure with added nutrients, they are much easier to grow on soils that have been well cared for.
Remove runners from the parent plants as these compete with fruit production. Runners can be kept for planting in subsequent seasons, but there is a high risk of perpetuating unhealthy non-productive plants. This risk is less if the original plants were sourced from high health stock.
Fruit should not be harvested until fully ripe. Only then will it have developed the full flavour that strawberries are famous for.
Modern strawberry plants are designed to fruit heavily and well for six
to eight months. Trials have shown that while plants can persist for many
years, fruit size and production with the best quality usually occurs
in the first 6-8 months from the first fruiting.
These principles apply just as much to home garden production. The value to the environment, for families and neighbours, applies just as much in a home garden situation. The programme that brings you this information has been developed to show the value of science to home gardeners and also to bring it alive by encouraging the use of Integrated Garden Management (IGM).
What does IGM mean for strawberries grown in the home garden?
Choose a variety known for its suitability for the area and resistance to pest and disease attack. For some areas, this may not be well known, but neighbours or garden centre staff may be able to suggest suitable varieties. Most modern varieties have good tolerance to virus diseases. Chandler, Seascape, Gabrielle and Sundae have been selected for their tolerance to pests and diseases. Chandler and Sundae are well suited to the North Island, while Seascape and Gabrielle are well suited to the South Island.
A scheme operates in New Zealand to produce high health plants. Most, but not all plants sold to home gardeners are sourced from this scheme. Only buy strawberry plants from a reputable supplier. Step one in producing healthy fruit is to start with healthy plants. Plants given by friends or bought from local stalls may look fine but may harbour viruses and carry pests and diseases. Keeping runners from your own plants will usually work for a few years provided the original plants were healthy, but once virus diseases infect the plants, new stock is necessary. All old plants should be destroyed before planting new ones to avoid disease spread.
Ideally, new plants should be established every year. While older plants will continue to fruit (a Lincoln University trial kept a commercial bed in production for 10 years!), fthe ruit size of modern varieties is far superior and pest and disease management is much easier in the first year of fruiting.
Plants can be grown from seed, but all modern varieties are hybrids and only a small percentage of seedlings will usually be as good as the parent. The only exception is the small alpine strawberry that does not runner but grows true from seed. It produces very small but highly aromatic, tasty fruit. It is not grown commercially but is sometimes seen in seed catalogues.
Root feeding insects
Grass grub can be a major problem with strawberries. This native New Zealand pest can eat the roots of strawberry plants to such an extent that plants can be simply pulled straight out of the ground with minimal effort. Surrounding lawn areas often provide the source for grass grubs. Management control is desirable, e.g., controlling the pest by encouraging birds and rolling at its source. There are few controls available once the pest has started on strawberry plants other than soil insecticides such as Diazinon. To be effective this should be applied between December and March.
Slugs and snails
This is potentially the most serious disease for strawberries. The disease carries over on old leaves, so starting with clean planting material and removing any dead and dying leaves and rotten fruit throughout the season is a key part of managing botrytis. Encouraging air movement around the plants will help prevent germination of botrytis as it needs high humidity.
The disease attacks dead and dying flower parts and ripening fruit. In dry areas such as the east coast of both islands in New Zealand, management control as mentioned above is usually sufficient to provide control of this disease. If chemicals are required, they need to be applied to open flowers to obtain protection of flower parts where the infection starts. Infection often starts under the calyx so coverage in this area is particularly important. Under wet and warm conditions, infection can occur quickly. Individual flowers last for just a few days but are susceptible to infection during this time.
Mycosphaerella leaf spot (frog’s eye)
This disease can be tolerated at low levels without causing loss of yield. However, any early infection can cause rapid build-up of the disease later, especially in humid conditions. High infection levels result in smaller fruit with less flavour. Infection can take from 10 days to 3 weeks to show up. Encouraging air movement around the plants will help prevent germination as this disease also needs high humidity. In dry climates, and if plants purchased were clean, then no control measures are usually necessary.
The disease can cause black spots on leaves and leaf stems and occasionally crown rot, but the main problem is black fruit rot. Some varieties are particularly susceptible to this disease and should be avoided. Most infection occurs from carryover from old strawberry plants. Removing old plants before planting new ones should ensure that the disease does not spread from these.
Spray only when it is necessary. Unnecessary spraying leads to resistance build-up and possible residue problems. It also costs more. Decisions about the need to spray should be based on what is happening in the strawberry plants, rather than what is written in a book, or what was done last year. The decision to spray should be based on knowledge of plant health status when the strawberries were planted, weather conditions, and what is expected in the near future. Pest and disease levels on the plants, determined by having a close look at them, and understanding what level can be tolerated before it affects plant performance should also be considered.
The principle should be to avoid chemical use where possible, but if they are required, use ones that meet the criteria mentioned above.
The best options for pest and disease control are in the early part of the season, before first strawberry flowering. Do not spray strawberries after flowering unless you are sure that the chemicals are totally safe to pollinating insects and people.