Plant Doctor Archive

Leafless begonias

We recently moved to the Marlborough Sounds and for the first time my begonias have very little foliage. However, they are coming into flower bud. Can you tell me what fertiliser they might need, and what position is best for them? They are presently in pots, new potting mix, and are in full sun.

Begonias will defoliate for several reasons — stress caused by lack of water, over-watering, excessive root disturbance when repotting, too much fertiliser, or mildew (downy or powdery) which begonias are susceptible to.

If it is mildew, spray with Bravo or Greenguard at 14-day intervals until plants show improvement. Make sure there is good air circulating through the plants as mildews enjoy humid conditions.

Water plants in the morning rather than at night. If it is watering, adjust watering according to the weather conditions.

If too much fertiliser is the problem, water the plant to leach out any excess for at least half an hour. Being in a pot, plants will react quickly to over applications of fertiliser.

Are these bedding begonias or tuberous begonias? Bedding begonias will grow happily in full sun or light shade; tuberous begonias like bright light, but not direct sunlight, and are best in light shade — although there are now tuberous varieties (Begonia solenia) that will grow happily in full sun.

Osmocote is the best fertisliser to use for plants in containers. Use at the recommended rate. However, if the plants have been recently repotted into new potting mix, they will not require feeding for at least three months.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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