If you’re gardening in zone 5b, knowing when to plant onions is the key to a successful harvest. Getting the timing right ensures your bulbs have enough time to grow large and sweet before winter returns. This guide will walk you through the optimal schedule for both seeds and sets, so you can plan your garden with confidence.
Onions are a cool-season crop, which means they thrive in the cooler temperatures of spring and fall. Planting them at the correct moment protects them from severe frosts and gives them the long daylight hours they need to form bulbs. Let’s break down the best dates and methods for your zone.
When To Plant Onions In Zone 5b
For most gardeners in zone 5b, the optimal window for planting onions is in early to mid-spring. The goal is to get them in the ground as soon as the soil is workable. This usually means a few weeks before your last average spring frost date.
Here is a simple seasonal calender to follow:
- Late March to Mid-April: Plant onion seeds indoors.
- Mid-April to Early May: Plant onion sets or transplants outdoors.
- Fall (October): Plant a second crop of sets for an early spring harvest.
Understanding Your Zone 5b Frost Dates
Your local frost dates are the most important factor. Zone 5b has an average last spring frost date around May 15th. The average first fall frost typically arrives around October 15th. Onions can tolerate a light frost, so you can plant them well before that last spring date.
Always check a local frost date calculator for your specific town, as microclimates can cause variations. A good rule is to aim for planting onion sets or transplants 4-6 weeks before your last expected frost. For seeds started indoors, begin 10-12 weeks before you plan to transplant them outside.
Choosing Between Seeds, Sets, and Transplants
You have three main options for starting your onions. Each has it’s own ideal planting time.
- Seeds (Started Indoors): Start seeds indoors in late March. This method offers the most variety and is very cost-effective, but it takes the longest.
- Sets (Small Bulbs): These are the easiest for beginners. Plant sets directly in the garden in mid-April. They establish quickly and are less prone to disease.
- Transplants (Seedlings): You can buy these or grow them from seed yourself. Transplant them into the garden in late April or early May, when they’re about the thickness of a pencil.
Why Timing Differs for Seeds vs. Sets
Onion seeds need a long, cool period to establish strong roots and foliage before bulbing begins. Starting them indoors gives them this head start. Sets, however, are already partially grown. Planting them too early in cold, wet soil can cause them to bolt (flower prematurely). That’s why sets go in a bit later, when the soil has warmed slightly.
Step-by-Step Outdoor Planting Guide
Once your timing is set, follow these steps for planting outdoors.
- Prepare the Soil: Choose a sunny spot with loose, well-draining soil. Work in several inches of compost or aged manure a week or two before planting. Onions need good nutrition but don’t like fresh, hot manure.
- Planting Depth & Spacing: For sets, plant them just deep enough so the tip is barely visible, about 1 inch deep. Space them 4-6 inches apart in rows 12-18 inches apart. For transplants, plant at the same depth they were growing in their container.
- Watering In: Water the bed thoroughly after planting to settle the soil around the roots or bulbs.
Signs You’re Planting Too Early or Too Late
It’s important to recognize the risks of bad timing. Planting in soil that is too cold and wet can cause the bulbs to rot. If you see sets becoming mushy or seedlings turning yellow, this might be the cause.
Planting too late, on the other hand, means the bulbs won’t have enough time to size up before the long, hot days of summer trigger bulbing. You’ll end up with very small onions. If you notice bulbs forming while the plants are still very small, you likely planted too late.
Fall Planting for Overwintering Onions
An advanced technique in zone 5b is fall planting. This involves planting specially grown onion sets in October, about 4-6 weeks before the ground freezes solid.
- They develop roots before winter, go dormant, and then shoot up first thing in spring.
- This gives you a harvest of green onions or early bulb onions in June.
- Use only varieties labeled for “overwintering,” as they are bred to survive the cold.
Mulch them heavily with straw after the ground freezes to prevent heaving from freeze-thaw cycles. This method requires well-draining soil to prevent rot over the winter.
Caring for Your Onions After Planting
Proper care after planting ensures good growth. Keep the soil consistently moist, but not soggy, especially during bulb formation. A layer of straw or shredded leaves mulch can help retain moisture and suppress weeds.
Onions are shallow-rooted, so hand-pull weeds carefully to avoid damaging the bulbs. Fertilize with a nitrogen-rich fertilizer early in the season, but stop feeding once the bulbs begin to swell, as too much nitrogen late can affect storage quality.
Dealing with Common Problems
Watch for onion thrips, tiny insects that cause silvery streaks on leaves. A strong spray of water or insecticidal soap can help. Ensure good air flow and crop rotation to prevent fungal diseases like downy mildew. If an onion sends up a flower stalk (bolts), harvest and use it immediately, as it won’t store well.
When to Harvest Your Onion Crop
Harvest time depends on whether you want green onions or storage bulbs. You can pick green onions anytime they reach a usable size. For full bulbs, wait until the tops begin to turn yellow and fall over naturally.
- Once most tops have fallen, gently lift the bulbs with a garden fork.
- Lay them out in a warm, dry, airy place to cure for 2-3 weeks.
- After the necks are tight and the outer skins are papery, trim the roots and tops and store them in a cool, dry place.
FAQ: Planting Onions in Zone 5b
Can I plant onions in May in zone 5b?
Yes, planting in early May is still acceptable, especially for sets or transplants. However, planting later than mid-May may not give long-day onion varieties enough time to form large bulbs before summer days trigger bulbing.
What is the best month to plant onion sets?
The best month to plant onion sets in zone 5b is April. Aim for the middle to end of the month, when the soil is no longer frozen and has begun to dry out a bit from spring rains.
Is it to late to plant onions in June?
In zone 5b, June is generally too late for planting onions intended for large storage bulbs. The days are already long, which will signal the plants to start forming bulbs before they have grown enough foliage. You might only get small bulbs or green onions.
Should I start onion seeds indoors?
Starting onion seeds indoors is highly recommended if you want specific varieties not available as sets. It gives you a much wider selection and can be more economical if you need a large quantity of plants. Just remember to give them plenty of light so they don’t get leggy.
Can onions survive a frost after planting?
Yes, established onion plants can survive a light frost. In fact, they prefer cool weather. A hard freeze (below 28°F) can damage young plants, so be prepared to cover them with row cover if an extreme cold snap is forecasted after you’ve planted.
By following this timing guide, you give your onions the best possible start in your zone 5b garden. With a little planning in the spring, you’ll be rewarded with a plentiful harvest of crisp, homegrown onions that will last you for months.