Q&A - Container gardening

Q. How and when should I prune an Acer planted in a large pot?

A. Most Acers should not really need pruning particularly the Japanese types. If you do want to do some selective pruning then this is best done in early spring before the leaves appear.

Q. I have lots of what can only be described as ugly green "splodges" on top of the compost in the tubs in my garden, they look like algae of some kind and always reappear when I pull them out. They are only in the tubs containing grasses and cordylines for some reason. Please could you tell me what they are and how to get rid of them?

A. Your problem sounds very much like an algae and is usually the result of wet conditions. Try removing the plants from their pots and knocking off about a third of the compost all round. Make sure you have plenty of drainage material in the bottom of the container (broken pots are ideal) and then replant using a well-drained gritty compost. In the winter stand the pots on feet, small bricks or pieces of wood to improve the drainage.

Q. I have received a lot of mixed advice lately on how to care for Bay Trees through the winter months in in UK (North East) climate, including keeping in the garage (no sun?), inside in well lit room (Central heating?) and keeping close to the house near the warmth of exterior walls (or with a light mesh/ net over them?). Can you advise whether any of these methods are appropriate or how I can keep them through the winter months to minimise the amount of damage prior to the spring?

A. The key to keeping your bay tree healthy during the winter in the North East where winters can be quite cold is to provide a place where the plant will not be exposed to low temperatures and very cold winds. The plant will require normal winter light conditions and the compost should be free draining (but kept moist) which can be achieved by standing the pot on pot feet or bricks. The ideal place is a cool greenhouse or conservatory that is regularly ventilated. Failing this you should find a sheltered spot by a wall where you can wrap the plant in fleece or bubble wrap and the pot in warm blankets or bubble wrap to stop the roots from freezing during extreme weather conditions. You will have to let air and light to the plant from time to time though to prevent any diseases building up and the plant "cooking".

Q. I have a large Datura which has lots of leaves with many shoots coming from the stems, I have fed it well with tomato food (every other watering) and it looks very healthy but no flowers, what am I doing wrong?

A. Your Datura needs plenty of sunshine and water during the summer to flower. They are best placed outside during this time and should be in a large pot.

Q. I have purchased a marguerite which bears white daisy like flowers which I regularly dead head. It is about 2 feet tall and the blooms all appear from the top of a single strong stem and is shaped like a small tree. I understand that this plant is a perennial but I don't know how to care for it over winter once the flowers have finished. Does the main stem die down with the blooms? Can I leave it planted in the garden?

A. Your Marguerite sounds like one which is generally grown as a half hardy summer patio plant as it will not generally survive outdoors in the winter. You can keep it over the winter provided you take it into a light frost-free spot such as a heated greenhouse or conservatory. It will need keeping cool and the compost only slightly moist. You may need to trim back the shoots by about 50% to keep it neat and tidy. You may get shoots appearing along the stem or from the base and these can be simply removed with a pair of sharp secateurs.

Q. I have been given two Osteospermum plants, one white, and the other purple. Can these plants withstand the winter months in the garden, or do they have to be brought indoors for the winter.

A. Most Osteospermum are not hardy and would need to be kept frost free during the winter. Also make sure the compost is only just moist otherwise the fleshy roots could rot. There are a couple of varieties though that will survive outside in some parts of the UK given the right conditions such as a sheltered spot and well drained soil.

Q. Please advise on the cultivation of Olea as an indoor plant in winter and as a patio plant in summer. I purchased a 6ft tree several weeks ago and it is shedding leaves.

A. Olives will grow outdoors in very mild parts of the UK but do well in conservatories and the home given the right conditions. The optimum temperature range is 5-25 degrees Celsius (41-77 F) allowing the plant to get as much warmth as possible in the summer either indoors or out. Grow in a container that is about 2.5cm (1") diameter larger than the roots and use a soil based compost that is well drained but holds moisture. Water regularly in the summer and keep the compost just moist in the winter. Feed with a general purpose fertiliser such as bonemeal, fish blood & bone or Rooster each spring. In summer apply a liquid feed every three or four weeks.

Q. I have just bought the above plant, which is in a hanging pot with small dark green leaves, and dark pink small flowers. It is a trailing plant, but unfortunately, has no guidelines as to how to care for it. Can you help me with any tips?

A. Portulaca grandiflora is know as the sun plant as the flowers open in the sun and close in shade. It is only half-hardy so needs protecting from frost and cold. It needs lots of light and can be prone to attack from aphids. It is really good in baskets where it will trail to a length of 15-20cm (6-8") with a similar spread. Keep it well watered and feed weekly with a liquid plant food such as MiracleGro or Phostrogen.