Native to the Mediterranean basin, olive trees, known botanically as Olea europaea, grow best in subtropical climates that boast warm, dry summers and mild winters. While the natural climate of the United Kingdom is largely unsuitable for growing olive trees, more British gardeners are trying their hand at growing olive trees indoors in greenhouses. If gardeners meet cultivation requirements, then greenhouse-based olive trees can grow successfully and produce olives so healthy and delicious that they rival their Mediterranean counterparts.
Buy a healthy, young olive tree from a local nursery or garden centre. Order olive trees from reputable catalogue or online nurseries if you are unable to purchase them locally.
Select an appropriate growing container for the olive tree. Choose a container that is large enough to house the roots of your tree while providing a couple of extra inches of room for growth. Make sure that the container you choose has at least four 1/4-inch holes in the bottom to allow for healthy drainage. Drill additional holes in the bottom of your growing container, if necessary.
Prepare your selected container for planting. Measure the bottom of the container with a measuring tape, and cut a wire mesh screen to those dimensions. Place the screen in the bottom of the planting container to prevent soil from washing out during watering. Top the screen with 1 to 2 inches of landscaping gravel to hold it in place.
Plant your olive tree in its growing container at the same level it was planted in its nursery container. Blend equal parts compost, peat moss, perlite and coarse builder's sand to create a fast-draining growing medium for your olive tree.
Situate your olive tree in the greenhouse, as desired. Heat the greenhouse in the winter, if necessary, to provide your tree with the temperatures it needs for healthy, vigorous growth. Keep the winter temperature of your greenhouse between -5.56 and 12.2 degrees Celsius to provide temperatures that are cool enough to induce dormancy, but warm enough to prevent frost damage.
Water your greenhouse olive tree regularly during the spring and summer to keep the growing medium moist, but well-drained. Allow the top inch of growing medium to dry to the touch between watering to avoid over-hydrating the olive tree. Water the olive tree less frequently in the fall and winter when the tree is entering dormancy. Provide just enough water to keep the growing medium from completely drying out.
Fertilise your olive tree with a water-soluble 20-20-20 fertiliser once every two weeks during the spring and summer. Reduce fertilisation to just once a month during the fall and winter months. Apply the fertiliser application according to label directives for best results.
Prune your olive tree at the end of winter, but before your tree starts to flower. Remove any discoloured, diseased, broken or dead branches to keep your tree looking and feeling healthy. Remove crossed or rubbing branches to increase air circulation. Minimise the pruning stress that your tree experiences by making clean pruning cuts with sharpened and sterilised pruning tools.
Move container-grown greenhouse olive trees outdoors in the summer, if desired. Return your olive tree to your greenhouse before the onset of cold fall or winter weather.
Monitor your tree for signs of infestation by common insect pests like the medfly and olive fruit fly.