When to Plant Onions in Idaho (2026 Guide)
Growing onions in Idaho can be a rewarding experience if you choose the right varieties and timing. Onions are a staple crop that take patience but reward you with storage crops that can last for months. Understanding your local frost dates, like the last spring frost of May 6 in areas like Boise, is crucial to successful planting.
Idaho's diverse climate and varying USDA hardiness zones mean that planting timelines differ significantly from region to region. For instance, areas in Zone 5a, such as Idaho Falls, see their last frost around May 25, enabling a shorter growing season of around 116 days. It is essential to align your planting schedules accordingly.
Whether you're starting from seeds or transplants, onions require careful attention. Remember that the success of your onion crop greatly depends on choosing the correct type based on your latitude. Long-day varieties are ideal for northern gardeners, while short-day types are reserved for warmer southern regions.
Planting Schedule by USDA Zone in Idaho
| Zone | Indoor start | Transplant out | Direct sow | Harvest |
|---|---|---|---|---|
| Zone 5b | Start February 15 – March 1, ~6–8 weeks before May 6 | Transplant outdoors from April 10 – April 20 | Direct sow seeds from April 10 – April 20 | Harvest from September 15 – October 1 |
| Zone 5a | Start March 1 – March 15, ~6–8 weeks before May 25 | Transplant outdoors from April 25 – May 5 | Direct sow seeds from April 25 – May 5 | Harvest from September 1 – September 15 |
| Zone 4b | Start March 15 – March 25, ~6–8 weeks before June 12 | Transplant outdoors from May 15 – May 25 | Direct sow seeds from May 15 – May 25 | Harvest from September 1 – September 10 |
Onions Planting Windows in Idaho
- Indoor start: Start your onion seeds indoors between February 15 and March 15, depending on your zone.
- Transplant outdoors: Transplant your onion starts outdoors between April 10 and May 25, guided by your local frost dates.
- Harvest: Onions can typically be harvested from late September to early October, depending on your zone's specifics.
Common Mistakes Growing Onions in Idaho
- One common mistake among Idaho gardeners is assuming all onion varieties will thrive without considering day length. It’s vital to choose long-day varieties in northern areas to avoid poor yields.
- Idaho's unpredictable late spring weather can result in sudden frost even after average last frost dates. Always monitor forecast changes and be prepared to cover plants if necessary.
- Some Idaho gardeners plant too deeply, forgetting that onions form bulbs above the ground. Proper planting depth is crucial, with roots just below the surface for optimal growth.
- Ignoring soil conditions is a mistake many Idaho gardeners make. Given Idaho's varied soil types, it’s crucial to amend with organic material to provide necessary nutrients for heavy-feeding crops like onions.
- Many Idaho residents underestimate the importance of timing their sowing and transplanting. Being too late in the season can lead to smaller bulbs and reduced harvest quality.
Frequently Asked Questions
What if I'm in a cold pocket of Idaho?
If you're situated in a colder area, consider starting your onions indoors earlier and using row covers when planting out. This protects against unexpected frost and extends your growing season.
Can I still plant onions in May?
Yes, if you're in Zone 5b or 5a, you can transplant onions in early May. Make sure to choose varieties that can mature during the available frost-free window.
When should I stop watering my onions?
It's best to stop watering when the tops of your onions start to fall over, which indicates they're nearing maturity. This helps the bulbs to cure naturally in the ground.
Why are my onion tops falling over?
Falling tops can indicate that your onions are reaching maturity, but they can also indicate stress due to water or nutrient issues. Ensure you are meeting their growing needs throughout the season.
What's the best way to store harvested onions?
After pulling your onions, cure them in a warm, airy place at 80–85°F for 2–4 weeks. Proper curing extends their storage life considerably.