Cutting the seed heads off onions is a common question for home gardeners. The seed heads that develop on top of onions are a sign that the onion plant is going to seed. This redirects energy away from bulb growth and into developing seeds, often resulting in smaller onions. There are pros and cons to removing onion seed heads that every gardener should consider.
Quick Summary
Cutting onion seed heads can help improve bulb size, but timing is important. Remove seed heads too early and you risk stunting bulb growth. Wait too long and the plant may have already redirected energy. Leaving seed heads can allow you to collect onion seeds, but seeds may hybridize. Overall, cutting seed heads is recommended in most cases to optimize onion bulb harvests.
What Are Onion Seed Heads?
Onion seed heads, also known as umbels or flower stalks, are part of the onion plant’s natural reproduction process. After an onion bulb reaches full size, the plant sends up a tall, narrow stalk from the center of the bulb. At the top of this stalk, a cluster of small white, pink, or purple flowers develop, eventually producing onion seeds if pollinated.
This seed head development requires a significant amount of energy from the onion plant. Resources get redirected from bulb growth into flowering and seed production. As a result, onion bulbs often stop increasing in size once the seed stalk appears.
Onion Flowers and Seeds
The tiny onion flowers that make up the seed head each contain male and female reproductive parts. When pollinated by insects, they develop into seeds over the course of the summer. Onion seeds are black, shaped like tiny teardrops.
Allowing onions to go to seed provides a way to collect seeds for future planting. However, onion flowers are prone to cross-pollination from other allium species like leeks, chives, and garlic. This can result in hybridized seeds that produce onions different from the parent plant.
Should You Remove Onion Seed Heads?
Whether or not to cut off onion seed heads depends on your gardening goals. Here are some of the key considerations:
Maximizing Bulb Size
Removing onion seed heads helps optimize bulb growth. By eliminating the draw on resources that flowers and seeds require, all energy goes to the onion bulb itself. Cutting the seed stalks can result in bulbs up to 25% larger compared to leaving seed heads intact.
Timing Matters
When seed heads are removed plays an important role. Cutting stalks too early may shock the plant and stunt bulb growth. Generally, it’s best to wait until bulbs are mostly formed before removing seed heads. This allows adequate foliage growth first.
As a rule of thumb, cut seed stalks when they are around 4-6 inches tall. This maximizes resources devoted to the bulb while preventing seeds from forming.
Collecting Seeds
Leaving onion seed heads in place allows seeds to mature for collecting and replanting. This provides free onion seeds for successive seasons. However, isolation from other alliums is needed to prevent hybridization.
Additionally, seed production draws energy away from the bulb crop. Onions left to seed will be smaller than those with stalks removed early.
Aesthetic Preferences
The appearance of the onion patch or garden may factor into the decision. Some gardeners prefer the tidy, uniform look of onions before seed stalks emerge. Others like the unique visual interest created by tall, vertical seed heads.
How to Remove Onion Seed Heads
Cutting off onion seed heads is a simple process. It’s best done by hand to avoid damaging the foliage. Follow these steps:
- Wait until seed stalks have grown 4-6 inches tall.
- Locate the narrow point where the stalk emerges from the leaves.
- Use garden scissors or pruners to snip the stalk off right at the base.
- Optionally, peel away any remaining dried leaves around the cut.
- Discard removed stalks or add to a compost pile.
It’s important not to cut the tops too soon, as this can reduce bulb size. Check onion plants frequently once bulbs start swelling and remove any emerging flower stalks at the right stage.
For large plantings, it may be easier to cut stalks rather than snipping individual seed heads. Simply run a sharp hoe or similar tool just under the soil surface to slice through all the seed stalks between onion rows.
When to Stop Cutting Seed Heads
Removing onion seed heads should stop once bulbs are fully mature. This is generally around mid-summer, depending on climate and onion variety.
At this point, the bulbs have reached maximum size and will not increase any further. Taking off additional stalks provides no extra bulb growth. Leaving late-developing seed heads intact lets plants fully ripen and prepares them for harvest and curing.
Signs Onions Are Fully Mature
- Foliage starts to fall over and dry up
- 40-50% of the foliage has turned yellow or brown
- Bulbs feel firm and filled out when gently squeezed
- Outer onion skins become thin and papery
Once most plants display these signs, bulb growth has stopped. Further removal of seed stalks is not necessary.
Do All Onion Varieties Produce Seed Heads?
Most common onion types will develop seed heads and flowers if left to grow. However, some varieties are more prone to bolting and going to seed than others.
Onion Types that Readily Bolt
- Yellow storage onions
- Red storage onions
- White storage onions
- Pearl onions
- Potato onions
Onion Varieties Less Likely to Bolt
- Yellow sweet onions
- Vidalia onions
- Walla Walla sweet onions
- Rampicante Italian cipollini onions
- Evergreen Hardy White bunching onions
Consider onion variety when planning your garden. Quick-bolting onions likely need seed heads removed for optimal harvests. Slower bolting types may reach full bulb size before flowering in some conditions.
Do Weather and Environment Influence Bolting?
Growing conditions play a major role in whether onion plants bolt and produce seed heads. Certain factors encourage onion bulbs to stop growing and switch over to flowering and seed production.
Conditions Promoting Onion Bolting
- High temperatures
- Long daylight hours
- Drought or water stress
- Interrupted growth from diseases, pests, or damage
- Low soil fertility
- Crowded planting
Hot summer weather is a common cause of onions prematurely shooting up seed stalks. Irrigating properly and providing balanced fertility can help minimize bolting.
Additionally, planting onion varieties suited to your climate is key. Choose early maturing, heat-tolerant types for hot regions to avoid excess bolting.
Companion Planting to Deter Onion Bolting
Certain companion plants are thought to help reduce onion flowering and bolting. Interplanting onions with these vegetable and herb allies can potentially optimize bulb production.
Recommended Onion Companion Plants
- Lettuce
- Beets
- Strawberries
- Carrots
- Parsley
- Dill
Pairing onions with shade-providing plants like lettuce may help deter early seed stalks. Herbs like dill and parsley may have biochemical effects that slow bolting as well.
Companion planting is not a guaranteed solution. However, it provides easy insurance against bolting in close-grown garden conditions.
Storing Onions After Removing Seed Heads
Cutting off seed heads has no impact on onion storage. Bulbs still need proper curing and drying whether stalks are removed or left intact.
After lifting bulbs, allow them to cure outdoors in dry conditions out of direct sunlight. Place onions in a single layer with good air circulation. Curing usually takes 1-2 weeks depending on humidity and air temperature.
Once thoroughly cured and dried, trimmed tops, brush off any soil, and store bulbs in a cool, dry spot. A basement, pantry, or refrigerator provides ideal onion storage conditions. They will last for months when kept at 32-50°F.
Can You Replant Onion Seeds?
Onion seeds collected from homegrown plants can certainly be replanted. However, there are some important considerations:
- Isolate seed producing plants from other alliums to prevent hybridization.
- Collect seeds as soon as they are fully dry on the stalks.
- Store seeds in a paper envelope or bag in a cool, dry location.
- Sow seeds thickly in early spring for best germination.
- Onion seeds are short-lived, so replant within 1-2 years for best viability.
Replanting onion seeds saves money compared to purchasing them. Just be aware that germination rates and growth characteristics may be less predictable compared to commercial seeds.
Other Uses for Onion Seed Heads
While most onion flower stalks get discarded, they can actually be put to use in several ways:
Cooking
Chopped onion stalks can be used to add subtle onion flavor to soups, stocks, and other dishes. Remove any tough outer layers first.
Crafting
The dried seed heads have an interesting architectural shape for using in dried flower arrangements, wreaths, and other crafts. Simply hang upside down to preserve form.
Fire Starters
Hollow dried onion stalks dipped in wax or paraffin make excellent fire starters for camping, fireplaces, or grills.
Compost
Chopped up seed heads add valuable nutrients and biomass to the compost pile or bin. They break down readily once returned to the soil.
Key Takeaways on Cutting Onion Seed Heads
- Cutting seed heads redirects energy to bulb growth, increasing yields.
- Wait until stalks are 4-6 inches tall for best results.
- Stop removing seed heads once bulbs are mature.
- Leaving some plants to go to seed provides free onions for replanting.
- Hot temperatures encourage premature seed stalks.
In summary, cutting off onion seed heads at the right time maximizes harvestable bulbs in most cases. But leaving some flowers may be useful for collecting onion seeds or achieving a decorative look. Consider your specific gardening aims when deciding whether to remove seed heads.
