Growing Strawberries in a greenhouse
Extra-early strawberries can be obtained quite easily by growing in pots in a heated greenhouse. Fruiting time is governed by when the pots are housed and the night temperature which can be maintained. A minimum temperature of 55°F (13°C) is desirable for ripening and, obviously, much more fuel is necessary to maintain this early in the year than in spring.
The old variety ‘Royal Sovereign’ is still preeminent for this purpose and the first essential is to secure really early-rooted runners. Pot-grown runners should be obtained in July if possible.
Plant them in 16cm (6in) pots using John Innes potting compost No. 3 and stand them outdoors, on a bed of ashes. The latter prevents worms entering the drainage holes and helps to prevent drying out. The pots should be in full sun and watered daily.
If the ash bed is deep enough for the pots to be plunged up to their rims, this will protect them from frost and they may stay there until it is time for them to go into the greenhouse. If the pots are exposed, however, it would be better to remove them in November, when growth has ceased, to some sheltered spot such as on the south side of a wall. Lay the pots on their sides and cover with straw to protect them from frost.
Given sufficient heat, plants housed in mid-December may ripen their fruit at the beginning of April and those brought in at the end of January should have ripe berries during the latter half of May.
The strawberry pots should be stood in the lightest position available in the greenhouse. At first, no artificial heat should be given and the temperature should be kept down during the daytime, if necessary by free ventilation. No water should be given but on sunny days the plants should be lightly syringed and the greenhouse atmosphere should be moist rather than dry.
After about two weeks, signs of growth should be observed and a first watering should be given. From now onwards keep the compost in the pots just nicely moist, judging needs by tapping the pots daily—a ringing sound indicating that the compost is dry, a dull sound that it is wet.
Once the plants are growing freely, the temperature may be permitted to rise very gradually. Aim at a minimum of 45°F (7°C) by night. On sunny days it may still be necessary to give extra ventilation to prevent an undue rise of temperature—liable to encourage the leaves rather than the blossom.
When the blossom opens, a slightly higher temperature is wanted—up to 50°F (10°C) at night—and syringing should stop. A buoyant atmosphere will help pollination but nevertheless, as insects are scarce inside a greenhouse at this time of the year, recourse should be had to hand fertilization of the flowers. Around midday, using a camel’s hair brush or a small wad of cotton wool, dab the centre of each flower in turn, thus transferring the pollen.
When the fruit has set and the blossom petals have fallen, night temperature may go up to a minimum of 55°F (13°C), a moister atmosphere may again be permitted and syringing resumed. This will help, too, to keep the red spider mites at bay. These are often a trouble with forced strawberries and fumigation with azobenzene smoke is the answer when the infestation is severe.
While the fruits are swelling, a little gentle feeding will be beneficial—weak liquid manure once every 10 days or a proprietary liquid fertilizer used according to the maker’s instructions.
Once colour shows in the fruit, all feeding must cease, and drier conditions must prevail or botrytis disease will be encouraged: stop syringing, keep the path drier and give more air. Never, however, allow the pots to dry out. In sunny spring weather, watering may be necessary twice a day.
To obtain the fruit of top size, such as those which win gold medals at Englands Chelsea Show, thinning is essential. Remove the smallest berries at an early stage and be content with nine berries per pot. Once you have your nine, remove all further blossom. Use little-forked pieces of twig or bent galvanized wire as props to hold the trusses up and keep the berries in the light and away from the soil or pot side.
Once the berries have been picked the plants are of no further use and should be burned to prevent the spread of red spider mites.