Growing Strawberries Under Glass
Strawberries are native to Europe in small-fruited forms, Fragaria vesca and several others. The cultivation of large-fruited is attributed to the introduction of Fragaria virginiana from North America in the 17th century, the first British raised large-fruited variety being Keen’s ‘Imperial’ by Marshall Keen in 1806.
Much research has gone into the techniques of growing strawberries successfully under glass, as they are a high value crop. It is highly important in the first place that runners be selected only from plants of the highest quality and as free as possible from all strawberry ailments, virus and eelworm in particular. Every effort must be made to get young runners established in their forcing grounds as soon as possible. Mobile structures are frequently used in Holland and Belgium for early strawberry culture and the young newly rooted runners are planted out in their respective plots directly they are severed from the parent plant and irrigated until they are growing strongly.
For mobile greenhouse culture the plants are planted at 25 x 25cm (10 x 10in) or 30-30cm (12 x 12in) or thereabouts in 1.35m (4ft 6in) beds, the greenhouse being pushed over the plots in December or January. The Belgians plant at the same density in beds 75-90cm (2ft 6in-3ft) and 60-90cm (2-3ft) apart in a static greenhouse in autumn. Black polythene mulch with lay-flat irrigation underneath the polythene is invariably used, and irrigation is once more relied upon to prevent any set back.
A more modernis the waiting bed technique when multi-crown plants are grown on outdoor beds, de-blossomed and planted under protection in sequence, generally three or six plants per Growbag, watered by trickle systems. On a still smaller scale the young rooted runners are potted into 13-15cm (5-6in) pots after severing from the parent plant, using John Innes No 2 compost or similar. Growbags can be used to excellent effect.
A vernalization period is thought beneficial for strawberries in order to initiate flower buds, and is brought about by periods of lower temperatures. In small scale growing the pots are left outside until mid to late winter, being protected from severe frost by straw or other means. The necessary vernalization period is completed by the time they are lifted into the greenhouse or covered. The plants are kept cool for 3-4 weeks and then heat is applied to coax them gradually into growth. Flowers will form according to the temperature level given, generally in late winter or early spring, during which time theshould be dusted with a rabbit’s tail or cotton wool to assist pollination. Strawberries are, in the main, self-fertile, but at that time of year and in the environment of a greenhouse there will be few, if any, pollinating insects, though high temperatures will encourage bees if a hive is placed in or near the greenhouse. If kept well watered and given weak liquid feeding the plants can be encouraged to produce fruit by mid to late spring, considerably earlier than out of doors. At lower heat levels they will still crop much earlier than out of doors.
Commercially they are only grown for one year in, new ‘maidens’ being planted annually, although two-year-old plants can be lifted when dormant and can perform extremely well. Strawberries ideally grow under NFT systems in “A” frames.
Cold frame culture
Strawberries can also be grown most successfully in cold frames, the runners being established in 25 X 25 cm (10 x 10in), the sashes being put on during late winter. Drying out or lack of pollination is the risk in frames, resulting in small malformed fruits. When possible the sashes should be left off during the day in the flowering period to allow entry of insects for pollination, or this can be done artificially by dusting the flowers, a practice essential with early flowering in heated frames. The plants can be left in frames for two or three years, but kept well cleaned of runners. When strawberries are grown under cloches, polythene tunnel or floating mulches the same general procedure is followed. It is important to avoid thebecoming ‘sick’ due to repetitive growing on the same site.
Environmental control is particularly important with strawberries grown in greenhouses, frames, cloches or tunnels, and it may be necessary to take preventive measures in wet humid areas by spraying with fungicides. Support for the fruit by wires or other means will help by preventing fruit/soil contact. Strawberries in greenhouses respond to low intensity artificial lighting at 9-10W per m2/sq yd given during the night for 2-3 hours, this encouraging earlier flowering.
Cold storage for runners
The putting of runners into cold storage will allow planting in early and midsummer for autumn cropping under glass, or alternatively the early establishment of plants in pots in late spring, de-blossoming by removing flowers and bringing well-established plants into the greenhouse in mid-winter. If when runners are lifted they are tied in bundles of 25 or 50 and stored long-term in 38mu gauge polythene bags at —2 to —1°C (28-30°F), this allows planting out over a period and provides for successional cropping. This is complementary to waiting beds.
It is possible to obtain good runners in July and August. As the little plants appear, peg them down into the soil or into 3-in. pots sunk into the soil using strong wires bent like a hair-pin. This will enable the roots to form more quickly. Plant out the newly rooted plants in freshly prepared beds. If they have been rooted directly into 3-in. pots they can be potted on in August and used for forcing under glass for earlier fruit.
In April every year apply afertilizer containing 10 per cent potash at 3 oz. per sq. yd. Immediately after fruiting give hoof and horn meal at 3 oz. per sq. yd.
When frost is expected, cover the strawberry bed with sheets of newspaper or polythene to protect the plants.
Since there is seldom any wind on a frosty night, the newspapers will not be blown away.
As birds are very partial to strawberries, put fish netting all over the plants early in the season to give protection.
Cambridge Favourite (Hartley Mauditt strain), mid-season. Large, salmon-scarlet. Heavy cropper, compact grower.
Cambridge Late Pine, late. Conical, crimson, sweet flavoured. Resistant to frost and.
Cambridge Prizewinner, early. Light scarlet, firm. Plants spreading, love wood ash at 5 oz. per sq. yd.
Cambridge Rival, early. Large, conical, crimson. Good variety for sandy soils.
Cambridge Rearguard, very late. Rounded, dark crimson. Very subject to red core disease and. Loves sandy land.
Cambridge Sentry, mid-season. Glossy crimson. Resistant to diseases. Good for bottling and jam-making.
Cambridge Vigour, early. Glossy crimson. Resistant to disease and frost.
Merton Princess, mid-early. Large, crimson. Susceptible to red core.
Red Gauntlet, mid-season. Large, scarlet. Vigorous compact grower. Large trusses.
Royal Sovereign, mid-season. Scarlet, large, delicious. Likes heavier soils.
Talisman, mid-late. Large crimson. Excellent for jam. Resistant to mildew. Soft in rainy seasons.
These tiny strawberries are much loved on the Continent. They can be raised in the greenhouse at a temperature of 55° F. (13° C.) from seeds sown in March in boxes of No-Soil compost or John Innes seed compost.
When seedlings are 1 in. high, prick them off, 1 in. square, into boxes containing John Innes potting compost No. 1.
Grow the plants on in the greenhouse on the shelving for another fortnight, and then harden them off in a frame until mid-May, when they can be planted out in rows l1 ft. apart, with 1 ft. between the plants. These plants will crop the same year.
Alternatively, plants can be obtained from a reliable nurseryman in September. Remove the first few flowers that appear, to encourage heavy cropping from August till October.
Baron Solemacher is the best variety to grow. It is a heavy cropper for very many weeks in the summer and is delicious as dessert and very good for jam. This variety never produces runners.
STRAWBERRIES IN BARRELS
If the only space available is a concrete yard or a veranda, it is still possible to grow strawberries—in a barrel. The barrel should be about 41 ft. high with a diameter of 3 ft. at the top.
Make four or fiveholes about l1 in. wide in the base of the barrel, and cover each hole with broken crocks. Then make 20 to 24 holes about 3 in. wide here and there in the sides of the barrel, through which the plants can grow. Gradually fill the barrel with John Innes potting compost No. 2.
As work proceeds, push the plants through the holes in the barrel from the outside, firming the soil over the roots. Insert six more plants in the top surface. Stand the barrel on bricks to ensure good drainage, and put it in a light position.
Water immediately after planting to help the soil to settle, and water regularly during the summer.
The best variety for this method of culture is Cambridge Favourite.
These are often known as perpetual straw-berries, and crop principally in September and October.
Prepare the ground as for ordinary strawberries but plant in October instead of August. Remove the first flowers completely; the second batch of bloom will fruit in the autumn.
Mulch the rows in mid-July with sedge peat, not only to keep the moisture in but to ensure that the berries are clean. The flower trusses are often long.
Plant firmly, 4 ft. apart, in rows 2 ft. apart, and keep the crowns of the plants just above soil level.
Hampshire Maid, August to November.
Round, firm, dark red. Good for jam, heavy cropper.
Red Rich, September to November. Dark red, juicy. Easy to grow.
Sans Rivale, September to December if covered with cloches. Large trusses, large, red. Very heavy cropper.
St. Claude, October to November. Solid, juicy, sweet. Dark green leaves. Resistant to disease.
St. Fiacre, September. Fair sized, red. Shortest cropping kind.
Note that Gorella and other similar types are especially good for early forcing and is one of the main varieties grown in continental Europe for this purpose. Royal Sovereign tends to be susceptible to botrytis under glass. Redgauntlet (early) and Talisman (late) can both be double cropped in a good year. For other varieties see specialist suppliers catalogues.