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Onions (from sets)

Allium cepa

INTRODUCTION

Growing Onions from sets (immature Onion bulbs) is an easier proposition than growing from seed and will save valuable time. Cultivation time will be far less than with seed raised plants but the range of available varieties to choose from is limited.

Growing Onions from sets is a great way for the beginner to get up and running quickly at a busy time of year on the vegetable plot. Sets are available from many seed merchants and are available for both autumn and spring cultivation periods.

Soil Preparation

Onions grown from sets are a very straightforward crop to grow but good soil preparation is essential for the best results.

The growing site should be in a sunny position with organic material added well in advance of sowing.

The ground should be light, fertile, free-draining and stone free. Heavy clay soils are not ideal.

Onions do not appreciate wet soils, especially if growing overwintered crops.

Onions also dislike acidic soils. Correct soil pH if necessary. 

Don't grow Onions in soil that has had Alliums grown in the last two seasons. Always practice crop rotation.

Sowing & Planting

Sets are available for autumn planting during late September and October, and again in spring from March onwards.

Each growing period has its own set of varieties specific to the time of year.

Autumn planted Onion sets will be ready for harvest earlier than spring-planted but are more prone to diseases from overwintering, particularly if the soil is wet for prolonged periods.

Growing techniques are similar for all set raised Onions.

Prepare the planting area by removing stones and weeds and rake to a fine tilth and gently firm soil.

Plant each set just below soil level with the tip showing and 10cm (4 inches) apart. Leave 25cm (10 inches) between individual rows.

As an alternative method, plant sets into modules or 9cm pots for planting out when weather permits.

Growing & General Cultivation

Keep growing area weed-free but take care not to damage roots when hoeing.

Keep watering to a minimum once the crop has established as Onions do not like to be overly wet. 

Feed once or twice during the growing period.

Pests & Diseases

Onions are in general, a relatively trouble-free crop but the occasional issue may arise:

Birds may pull sets out of the ground whilst searching for food. Simply replant. Covering crops with fleece cloches will eliminate the problem.

Bolting is the process where the Onion bulb prematurely flowers and runs to seed. It is caused by fluctuating growing conditions and temperatures. Try to keep growing conditions as constant as possible. If this becomes a regular problem, try growing Onions from heat-treated sets (The heat treatment removes flower embryos).

Onion White Rot is a soil-borne fungal disease and a serious problem if you have it. It will affect all members of the Allium family and can persist in the soil for twenty years or more. Avoid growing any Onion related crops in the same soil. 

Onion Fly will result in plants collapsing. The larvae eat roots in early summer, particularly during very dry weather. Cover susceptible growing areas with fleece.

Downy mildew which shows as yellow patches on foliage and affected leaves should be removed. To prevent, ensure good air circulation and low humidity. It is more prevalent in new growth. 

Harvesting

Autumn planted Onion sets should be ready for harvesting during mid-summer and spring-planted crops during late summer.

Bend over foliage as it starts to go yellow and leave to die off naturally.

When foliage has died down, lift Onion bulbs and allow them to dry in situ. Finish drying and curing in the greenhouse or shed.

Properly dried Onions should store well into the new year if kept frost-free.

Store in Onion nets or tied into Onion strings.

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