Getting your timing right is the first step to a great onion crop in Missouri. Knowing exactly when to plant onions in Missouri makes all the difference between small bulbs and a bountiful harvest. Our climate has distinct seasons, and onions are sensitive to day length. This guide will walk you through the best dates, varieties, and methods for success in your garden.
When to Plant Onions in Missouri
This is your core calendar. Missouri spans USDA hardiness zones 5b to 7a, so your specific location matters. Onions are typically planted in early spring, as soon as the ground can be worked.
Key Planting Windows
For most of Missouri, the ideal planting period is from mid-March to mid-April. You want to get them in the ground early because they need cool weather to establish roots and foliage before the heat of summer triggers bulb formation.
- Southern Missouri (Zone 7a): Aim for early to mid-March.
- Central Missouri (Zone 6): Target late March to early April.
- Northern Missouri (Zone 5b): Best to wait until mid to late April.
A good rule of thumb is to plant as soon as the soil is no longer soggy and has reached at least 50°F. You can check with a simple soil thermometer.
Fall Planting for Overwintering
Some gardeners have luck with fall planting, especially in southern Missouri. This involves planting onion sets or seedlings in October. They establish roots before winter, go dormant, and then resume growth very early in spring, often leading to an earlier harvest. It’s a bit more risky in colder regions without good mulch for protection.
Understanding Onion Types: Short-Day vs. Long-Day
This is crucial! Onions form bulbs based on day length. Missouri sits in a transitional zone, but most of the state does better with long-day or intermediate-day varieties.
- Long-Day Onions: Need 14-16 hours of daylight to bulb. Perfect for northern and central Missouri. Plant in spring.
- Intermediate-Day Onions: Need 12-14 hours of daylight. A safe and excellent choice for all of Missouri. Plant in spring.
- Short-Day Onions: Need 10-12 hours and are better suited for southern states. They may bulb too early and produce small bulbs if planted in spring here.
Choosing Your Onion Starts: Sets, Transplants, or Seeds?
You have three options, each with pros and cons.
- Sets (Small Bulbs): Easiest for beginners. Plant them directly in the garden in spring. However, they are more prone to bolting (flowering prematurely), which ruins the bulb.
- Transplants (Seedlings): These are young plants started from seed indoors. They offer the widest variety choice and are very reliable. You can buy them or start your own 10-12 weeks before your outdoor planting date.
- Direct Seeds: You can sow seeds directly in the garden in very early spring, but they take the longest to mature. Starting seeds indoors is generally more effective for a full-season grow.
Step-by-Step Planting Guide
Follow these steps for healthy establishment.
1. Site and Soil Preparation
Onions need full sun—at least 6-8 hours daily. The soil must be loose, well-draining, and fertile. A few weeks before planting, work in several inches of compost or aged manure. Onions are heavy feeders, so also mix in a balanced fertilizer according to package directions. Remove any rocks or debris that could obstruct bulb growth.
2. How to Plant Correctly
Proper spacing and depth is key.
- For sets: Plant them just deep enough so the tip is level with the soil surface, about 1 inch deep. Space them 4-6 inches apart in rows 12-18 inches apart.
- For transplants: Trim the tops back to about 4 inches tall to reduce transplant shock. Plant them about 1 inch deep, or so the white part is buried, with the same spacing as sets.
- For seeds: Sow them ½ inch deep and thin seedlings later to the proper spacing.
Water them in thoroughly after planting to settle the soil.
Caring for Your Growing Onions
Consistent care leads to bigger, sweeter bulbs.
Watering and Weeding
Onions have shallow roots and need consistent moisture, about 1 inch of water per week. Water deeply, especially during dry spells in late spring. Avoid overhead watering if possible to prevent foliar disease. Keep the bed meticulously weeded, as onions don’t compete well for nutrients.
Fertilizing Schedule
Feed your onions every 2-3 weeks with a nitrogen-rich fertilizer until the bulbs begin to form. Once you see the soil start to crack and push away from the forming bulb, switch to a low-nitrogen, high-phosphorus fertilizer to support bulb development. Stop fertilizing altogether once the bulbs have fully expanded.
Pest and Disease Watch
Common issues in Missouri include thrips (tiny insects that cause silvery streaks) and onion maggots. Use row covers early in the season to prevent maggot flies. For diseases like downy mildew, ensure good air circulation and avoid wet foliage. Rotate your onion bed each year to prevent soil-borne problems from building up.
Harvesting and Curing for Storage
Harvest time is usually late July through August. The signal is when about half the tops have fallen over and turned yellow-brown. Gently loosen the soil and lift the bulbs.
- Lay them or hang them in a single layer in a warm, dry, airy place out of direct sun for 2-3 weeks. This is called curing.
- Once the necks are completely tight and dry and the outer skins are papery, trim the roots and cut the tops back to about 1 inch.
- Store them in a cool, dry, dark place in mesh bags or baskets. Properly cured long-day onions can last for many months.
Frequently Asked Questions (FAQ)
What is the best month to plant onions in Missouri?
For spring planting, March and April are the primary months. Southern gardeners start in March, while northern gardeners should aim for April.
Can you plant onions in the fall in Missouri?
Yes, in southern parts of the state, you can plant hardy varieties in October for a very early summer harvest. Use mulch for winter protection.
What onion varieties grow best in Missouri?
Stick with long-day or intermediate-day types. Excellent varieties include ‘Copra’ (long-day, great keeper), ‘Yellow Sweet Spanish’ (long-day), ‘Redwing’ (intermediate-day, red), and ‘Candy’ (intermediate-day, sweet).
How long do onions take to grow?
From transplants, onions typically take about 90-110 days to mature. From sets, it’s slightly quicker, around 80-100 days. Growing from seed takes the longest.
Why are my onions so small?
Small bulbs can result from planting the wrong day-length type, planting too late, overcrowding, inconsistent watering, or insufficient nitrogen during the early growth stage. Getting the timing right—when to plant onions in Missouri—is often the root cause.
With this plan, your Missouri garden can produce a plentiful supply of onions. Remember to choose the right type for our daylight, get them in the ground early in spring, and provide steady care. A little attention to timing leads to a full season of flavor and a pantry stocked with homegrown onions.