Growing a successful crop of onions in Maryland requires aligning your planting schedule with the state’s variable climate. The precise timing is important because onions are extremely sensitive to day length and temperature cues. Understanding these requirements ensures the plants focus energy on developing a large, edible bulb rather than prematurely flowering or remaining small. Maryland gardeners must pay close attention to the calendar and local conditions.
Selecting the Right Onion Type
Onion varieties are classified based on their photoperiod, or the number of daylight hours required to trigger bulb formation. Since Maryland sits in a transitional zone, gardeners must select either Intermediate-Day or Long-Day varieties.
Intermediate-Day onions begin to form bulbs when the day length reaches 12 to 14 hours. These are the most reliable choice for the central and southern parts of the state.
Long-Day onions require a minimum of 14 to 16 hours of daylight to bulb, making them a better fit for the cooler, more northern and western regions of Maryland. The longest summer days in the state approach 15 hours, meaning varieties requiring 16 hours may not fully mature. Examples of suitable Intermediate-Day varieties include ‘Candy’ and ‘Superstar’, which are known for their good performance in the Mid-Atlantic. Planting the wrong photoperiod variety will result in small, underdeveloped bulbs regardless of planting date.
Timing the Main Spring Crop
The primary planting window for bulb onions is very early in the spring, as the plants need a long, cool period for leafy top growth before bulbing begins. Onion sets and transplants should be planted as soon as the soil is workable, typically four to six weeks before the average last frost date. This window generally falls between late February and the end of March across much of Maryland.
Onion transplants and sets are surprisingly cold-tolerant and can withstand a light frost after planting. The goal is to establish a robust root system and plenty of green tops before the summer’s longer days trigger bulbing. Gardeners who choose to start their onions from seed must sow seeds indoors in January to produce transplants ready for the garden in late winter. Planting too late compresses the vegetative growth period, resulting in smaller final bulb sizes.
Planting Onions for Overwintering
A secondary planting opportunity exists in late summer for an early harvest the following spring. This overwintering method is less common for large bulb onions in Maryland but is highly effective for multiplier onions, shallots, and Egyptian Walking Onions. The ideal time for this planting is late August or September, allowing the plant to establish a healthy root system before the onset of cold weather.
The plants will go dormant during the coldest months and resume growth quickly as soon as temperatures rise in late winter. Maryland gardeners attempting this method should be aware of the risk of bolting. Exposure to prolonged cold temperatures can cause the plant to produce a seed stalk in the spring instead of a large bulb.
Essential Site Preparation
Onions are heavy feeders with shallow roots, so the planting site needs careful preparation. They require a location that receives full sun, meaning at least six to eight hours of direct sunlight daily, to maximize the energy available for bulb development. The soil structure must be loose, well-drained, and rich in organic matter to accommodate the expanding bulb and prevent rot.
Prior to planting, amend the area generously with aged manure or compost to improve fertility and drainage. Onions perform best in soil with a slightly acidic to neutral pH, ideally ranging from 6.0 to 7.0. Ensuring the planting bed is free of weeds is important, as young onion plants cannot compete effectively for water and nutrients.