Planting Guides

When to Plant Onions in Boise: Complete Guide + Best Varieties for Zone 6a

Boise, Idaho

USDA Zone 6a

Last Frost: May 5

Last updated: October 30, 2025

Plant onions in Boise from mid-March through early April for summer harvest, or try fall planting in September for earlier crops. Zone 6a's 158-day season suits both intermediate and long-day varieties perfectly.

LLily Nakamura

October 30, 2025

Gardener planting onion transplants in Boise garden with Idaho foothills background in early spring — — Image © PlantReference.org 2026

Quick Answer

Plant onion transplants in Boise from March 15-April 10, choosing intermediate-day varieties for reliable bulbs. Harvest begins in August.

TL;DR

Plant onions in Boise from March 15-April 10 for summer harvest, choosing intermediate-day varieties like Allium cepa 'Candy' or long-day types like Allium cepa 'Walla Walla'. Zone 6a's 158-day growing season supports both spring planting (harvest August-September) and experimental fall planting with overwintering varieties. Plant garlic cloves in mid-October, 4-6 weeks before ground freezes, choosing hardneck varieties like Allium sativum 'Music' for Boise's cold winters.

Frequently Asked Questions

When exactly should I plant onions in Boise for the best results?

Plant onion transplants from March 15-April 1 in Boise for optimal results in Zone 6a. This timing allows establishment before your May 4 last frost date while giving plants maximum growing time before summer heat triggers bulbing. Transplants are more reliable than sets and offer better variety selection for Boise's intermediate-day requirements.

What's the difference between intermediate-day and long-day onions for Boise gardens?

Intermediate-day onions like Allium cepa 'Candy' begin bulbing when daylight reaches 12-14 hours, making them perfect for Boise's 44°N latitude. Long-day varieties need 14-16 hours and work at your location but have a shorter window for bulb development. Intermediate-day types offer more reliable harvests and better storage potential in Boise's climate conditions.

Can I plant onions in fall in Boise like southern gardeners do?

Advanced Boise gardeners can experiment with fall-planted overwintering varieties like Allium cepa 'Walla Walla' planted in September with heavy mulch protection. However, spring planting from March-April remains the recommended method for Zone 6a. Fall planting requires specific overwintering varieties and careful winter protection that most gardeners find challenging.

Why do my onions bolt instead of forming bulbs in Boise?

Bolting typically results from planting large onion sets (bigger than a dime), temperature stress during variable spring weather, or accidentally planting short-day varieties that won't bulb at Boise's latitude. Choose small sets or transplants, protect from late cold snaps, and verify you're planting intermediate-day varieties suited to Zone 6a conditions.

When should I harvest onions in Boise and how do I cure them properly?

Harvest begins in mid-August when 75% of tops fall over naturally—don't bend them manually. Boise's dry climate advantages curing: spread harvested onions in single layers in your garage or shed with good air circulation. The low humidity typically cures bulbs in 2-3 weeks until necks feel completely dry and papery when pinched gently.

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Tagged with

#onions #Boise gardening #Zone 6a #intermediate-day onions #garlic planting #Idaho gardening #mountain valley growing #spring planting #harvest timing #soil preparation

Written By

Lily Nakamura

Lily is a Master Gardener and high-desert homesteader outside Boise. She and her husband moved to Idaho from the Bay Area and had to completely relearn gardening for a climate with alkaline soil, intense sun, hard freezes, and limited water. Lily now grows a large vegetable garden, maintains a small orchard of cold-hardy fruit trees, and raises cut flowers for the local farmers market. She writes about the specific challenges of intermountain gardening—short seasons, temperature swings, and the importance of soil amendment in high-pH soils. Lily is honest about the learning curve: high-desert gardening is humbling, and she thinks more gardening writers should admit when conditions are genuinely difficult.