Sowing and Growing Main Onions (Bulbing)

What are Onions?

Onions belong to the allium family, which includes salad onions, over-wintered onions, main-crop onions, leeks, and garlic. This guide focuses specifically on main-crop onions, which offer a fresh harvest in late July and August and can be stored until around May. They can also be pickled or frozen to extend their availability even further.

Why Grow Onions?

At Wellies and Waxjackets, a rating system is used to evaluate the crops, considering factors such as taste, health benefits, cost, harvest yield, and availability in organic form. Main-crop onions rank highly, and spring onions are even better since they can be harvested fresh year-round. Here’s why onions are a great choice:

1. Delicious Flavour: Onions add a wonderful taste to many dishes.
2. Nutritional Value: They are extremely healthy.
3. Good Harvest: They yield well and store or freeze effectively.

However, they do have some downsides:

1. Readily Available: Onions are inexpensive and easy to find in shops.
2. Organic Options: They are often available in organic varieties.

How Many to Plant

For those accustomed to purchasing onions from the supermarket, it’s important to note that small to medium-sized bulbs typically grow at a density of 70 per square metre. Onions are harvested in August and can last until at least March, providing a supply for about eight months. To determine how many square metres are needed, calculate the number of onions consumed each month, multiply by eight, and divide by 70.

If space is limited, growing salad onions is recommended as they can be harvested fresh throughout the year and offer more value.

When to Sow

Onions are a staple in many households and are consumed year-round, which can be challenging since stored onions tend to rot or sprout in spring, while main-crop onions won’t be ready until late July. Wellies and Waxjackets employs two strategies to manage this:

1. Big Salad Onions: Used in place of bulbing onions during April and May.
2. Multiple Sowings: Including over-wintered varieties to ensure a consistent supply of bulbing onions starting from late May.

Growing from Seed or Set

At Wellies and Waxjackets, the preference is for growing from seeds rather than sets due to several advantages:

1. Cost-Effective: Seeds are generally cheaper.
2. Variety: A wider selection of onion varieties is available.
3. Better Storage: Onions grown from seeds tend to keep better.
4. Healthier Plants: Side-by-side trials consistently show larger, healthier plants from seeds.
5. Reduced Risk of Bolting: Seeds typically have a lower chance of bolting, though heat-treated sets are an exception.
6. Higher Planting Density: Easier to achieve with seeds.
7. Progressive Harvesting: Seeds allow for harvesting as salad onions, pickling onions, and eventually main-crop onions.
8. Backup Option: If seeds fail, sets can still be purchased, but if sets fail, it’s much more problematic.
9. Greater Satisfaction: Growing from seeds is more enjoyable and rewarding.

The most crucial factor is control. When growing from seeds, planting can be timed according to the long-range weather forecast, ensuring seedlings are established in favourable conditions. Sets, on the other hand, have less predictable growth conditions.

If sets are used, they should ideally be started in module trays, allowing for a strong root system to develop before hardening off and planting outside.

Sowing and Planting

Objective (Size/Yield)

When growing onions, it’s essential to define what the goal is. If the aim is to grow large onions for display, then the advice may differ. The focus at Wellies and Waxjackets is on ease, flexibility, resilience, continuity of harvest, and yield. Aiming for maximum size is more complex and typically yields less, but it can be more satisfying and provide a greater challenge.

Multi-sowing

Multi-sowing offers numerous benefits for onion cultivation:

1. Consistent Survival: At least one plant typically survives, ensuring no gaps in the planting.
2. Staggered Harvest: A variable number of seeds germinate per module, resulting in a staggered harvest and varied sizes. For example, modules with four onions often yield a mix of pickling and storage onions.
3. High Planting Density: Easier to achieve than with single sowing.
4. Early Transplanting: The root ball fills the module sooner, allowing for earlier transplanting or later sowing.
5. Resource Efficiency: Uses less compost and fewer trays, saving time.
6. Optimal Use of Space: Minimises the need for grow lights and sunny windowsill space.
7. Water Efficiency: Higher density planting results in less overall water usage.
8. Pest Management: Reduced nematode application when protecting against onion flies due to higher planting density.

However, there are some disadvantages:

1. Resource Competition: Plants may compete for nutrients and water, especially in cooler seasons when organic nutrients are less available.
2. Variable Growth Rates: Particularly relevant for spring onions and beetroot, where the aim may be to clear a bed at once, though this has less impact on main-crop onions.

Sowing, Pricking Out and Potting On

Wellies and Waxjackets recommend sowing onions and shallots in 40 cell module trays, with seeds sown 1cm deep and top watered until the onions are about one month old and thriving. For most onions, four seeds are sown per module, aiming for three to germinate. Larger varieties, such as Ailsa Craig, may require only three seeds per module, hoping for two to germinate.

When germinating indoors, additional heat isn’t necessary. Wellies and Waxjackets prefers to germinate alliums in an unheated bedroom at 16-18°C, where they typically take around 10 days. After germination, they are moved to a bright windowsill.

A note about grow lights: in January and February, seedlings benefit from a few weeks under lights post-germination. When sowing in February, a bright windowsill is sufficient until March, when they are transitioned to an unheated polytunnel or greenhouse. If neither is available, keep them in a bright area and consider moving them outside on sunny days for full sun exposure.

For those germinating in a greenhouse, shed, or polytunnel, some additional heat is advantageous.

Wellies and Waxjackets never pots on or pricks out. Seeds are sown in two batches, early and mid-February, and planted out in early April when they are 8-10 weeks old. The plants are small but possess a robust root system that holds the compost together in the module.

Onions do not respond well to repeated freezing, so maintaining a temperature above freezing with fleece is recommended.

Watering Seedlings

Alliums are top watered until they are growing vigorously and absorbing a significant amount of water from their modules daily. This prevents them from sitting in overly wet compost while young, ensuring maximum oxygen exposure and reducing the risk of root rot.

Trimming Seedlings

While onions can be grown without trimming, many professionals advocate for this practice as it encourages stronger growth. This approach is particularly beneficial for onions sown early, which can be long and thin. A trial of trimming is planned for 2021, especially as it may help onions establish in windy conditions. If planting in early April, trimming in early March appears to be an optimal timing.

Research indicates that trimming may reduce bulb size and increase disease risk, leading to a reconsideration of this practice.

Feeding Seedlings

Seedlings in modules packed with compost typically do not require feeding until they are planted out. However, if sown in January and planted in mid-April, they may lack nutrients. A diluted seedling feed, such as a mix of seaweed and fish emulsion, is advisable.

Planting Density

The ideal goal is to grow alliums to a medium size. This strategy reduces splitting, increases yield, simplifies watering, minimizes nematode usage for controlling onion fly, and facilitates drying while taking up less bed space. However, this ideal is rarely achieved—approximately 10% of onions will be large, 20% small, and 70% medium, which Wellies and Waxjackets embraces.

Small onions are pickled, large onions are used first or frozen, and medium-sized ones are stored. Any that go to seed are utilized in cooking, with the bulb chopped and frozen.

Considerations for planting density include:

1. Onions require ample water; lower densities should be used in drier conditions.
2. Onions thrive in rich soil (not overly nitrogenous); reduce density in poorer soils.
3. Lower density leads to larger onions.
4. Higher density can yield more, but space limitations may necessitate denser planting.
5. The risk of pests and disease increases with higher density, but it also allows for easier protection measures.

Wellies and Waxjackets prefer to over-sow and thin their onions, sowing roughly twice the needed amount and removing the excess as spring onions.

For sandy soil, which has remained undisturbed for five years and had ample compost, seaweed, and rock dust added, the following densities are used:

1. 70-80 main-crop onions per square metre, typically in clumps of three, spaced approximately every 7 inches.
2. 80 Zebrune shallots per square metre, also in clumps of three.

Based on 2021 trials, planting densities have been slightly reduced for more reliability. Increased rainfall or better soil conditions may allow for higher densities.

1. 70 over-wintered onions per square metre, ensuring care when interplanting.
2. 300 salad onions per square metre, typically interplanted rather than having dedicated beds.
3. 36 leeks per square metre, with potential for higher density with summer leeks.

A Few Points on Over-Wintering Onions

Onion varieties for over-wintering begin to bulb when day length exceeds 14 hours, typically mid-April. As onions sown in January are planted in early April, they often lack sufficient leaf growth for bulbing by then. Therefore, sowing in July or August and planting in October is recommended.

Over-wintering onions can be challenging; sowing too early may lead to excessive growth and bolting, while late sowing results in weak plants. Late July is advised for northern UK regions, while early August is suitable for the south. However, two batches are often sown since bolting isn’t catastrophic; affected onions can be frozen or eaten fresh.

For over-wintering, two batches of Toughball are sown directly into 40 cell trays, four seeds per module, aiming for three to germinate. The first batch is sown in mid-July, the second in early August, and both are planted out in early to mid-October, with some planted outside and some in the polytunnel for earlier harvests.

A Few More Tricks for Bigger and More Reliable Harvests

Plant in Multiple Locations

To mitigate risks from pests or diseases, Wellies and Waxjackets plants in multiple locations. For example, main-crop onions are grown in one main bed, a smaller bed on another allotment, and a crop along the front garden border.

Avoiding Bolting

Bolting is a significant concern for onions, particularly on sandy soil. Key factors to consider include:

1. Over-wintered red onions and early-planted red onion sets are most susceptible to bolting; planting later can help mitigate this.
2. Sets are more likely to bolt than onions grown from seed. For sets, heat-treated varieties are less prone to bolting.
3. Stressed onions, often due to heat or lack of water, are at risk of bolting; ensure adequate watering and consider reducing planting density.
4. Choosing bolting-resistant varieties and diversifying plantings can help manage risks.

Interplanting

Wellies and Waxjackets frequently interplant; salad onions are included in salad beds, onions are grown with parsnips, winter leeks with winter brassicas, and garlic is planted throughout, including in strawberry beds.

Progressive Harvesting

For those aiming for larger onions while maximizing yield, progressive harvesting is effective. Start by removing every other clump or plant, then continue this process. This can also be achieved by selecting varieties that serve dual purposes, such as those suitable for both salad and bulbing.

Weed Control

Effective weed control is crucial when planting at high density. Here are the steps followed:

1. Thoroughly weed the bed and provide ample watering.
2. If time permits, cover the bed for a couple of weeks with landscape fabric or tarp.
3. Remove the tarp, weed again, and eliminate any slugs encountered.
4. Apply amendments as needed.
5. Lay down an inch of spent mushroom compost as mulch to deter weed germination.
6. Plant through the mulch, ensuring the module surface is level with the soil.

Planting

Wellies and Waxjackets prefers to module sow all alliums, following these steps for onions, shallots, and salad onions:

1. Use a fork or spade handle cut to a point slightly larger than the 40 cell module tray.
2. Dib a hole 1 inch deeper than the module.
3. Water the hole.
4. Place the module into the hole, ensuring it sits about ½ inch below the module surface.
5. Gently firm the soil around it.

For leeks, the same process is followed, but the hole is twice the depth of the module. For garlic, the dibbed hole should position the tip of the garlic clove 1 inch below the surface of the mulch.

Watering

Onions require consistent watering, especially in spring when rain typically suffices. However, once bulbing begins in mid-April for over-wintered varieties or mid-May for main-crop onions, hand watering may be necessary. Onions thrive on additional water, but excessive moisture can lead to splitting and rotting. In the final month, watering should be adjusted based on bulb size and weather. For medium-sized bulbs, reduce watering; for smaller ones, provide slightly more. Cease watering two to three weeks before harvest while ensuring soil moisture remains.

The RHS suggests about an inch of water every couple of weeks, which is appropriate for sandy soil. While onions can survive drought, yield will be adversely affected.

Protecting from Pests

To safeguard young onions planted in April, protection with fleece or plastic is essential. While this helps guard against onion fly, Wellies and Waxjackets remains cautious about relying solely on mesh, as past experiences have shown mesh may not be entirely effective. Strategies for pest protection include:

1. Planting in multiple locations.
2. Interplanting to create diversity.
3. Protecting main beds with mesh.
4. Watering with nematodes, specifically using a product that protects against alliums, carrots, brassicas