Planting Seeds
Planting Potato Onion Seeds
When growing potato onions from seed it is 120+ days to maturity, and it is recommended to start seeds indoors 10-12 weeks prior to your last frost date. For Wisconsin, the last frost is April 30th, which means I should plant seeds between January 30 and February 15.
For the 2024 season I started seeds from four different providers in the basement on December 29, 2023 and January 24, 2024.
When planting potato onions from seed, it is a two-year project. Almost all onions, including potato onions, are biennials, therefore, when you plant a potato onion seed, the first year you will get a single bulb. That bulb needs to go dormant (i.e., curing) and the second season that it's planted is when the mother will clone itself into a nest of daughter bulbs. Like all onions grown from seed you can eat the first-year bulbs; just remember to save some bulbs to plant the second year so they multiply like true potato onions.
Why plant from seed? Over decades of cloning, the bulbs tend to become smaller, and sometimes develop diseases. Since daughter bulbs are exact genetic copies of their mother, all traits, viruses, etc. are passed on. The only way to reinvigorate a crop is to start over from seed.
One downside to cloning is that the more successive decades/generations down from the original mother bulb that was grown from seed, the less likely the bulbs will flower, and if they do flower, most likely they will be male-sterile. I'm guessing the original yellow potato onion bulbs I purchased from Southern Exposure Seed Exchange in 2020 are many generations old based on the fact that they have been producing sterile umbels (no seeds). That is why I've started purchasing other varieties of bulbs and true to seed. Introducing genetic diversity into my gardens should keep at bay potential problems with disease and smaller outputs.
Seed Viability: Onion seeds have a short storage life, and germination declines rapidly after a year at room temperature. When stored below 40°F (4°C), without freezing, they retain good germination up to three years. Seeds have no dormancy, so they can be sown as soon as they are harvested.
Starting Seeds Inside
Prepare your soil in pots or trays, scatter the seeds, gently press them into the soil and cover with 1/8 inch soil. I use 10x20 inch (25x50 cm) seed trays. Truthfully, there is no wrong way to plant potato onion seeds as long as the planting depth is shallow. They can be planted in divots, furrows, or spread haphazardly across the top. Seedlings don't mind being compacted tightly together, although, roots tend to entangle when grown too close together, making transplanting a tad more challenging.
Moisten soil with a spray bottle and cover trays with a clear dome lid or plastic wrap to help keep the moisture in.
Place trays in a warm location. Optimal temperature is 68-77°F (20-25°C).
Using a spray bottle, keep soil moist, not wet, until seeds germinate, which may start in 7-10 days, but could take up to three weeks. Do not expect all seeds to grow; 60% germination is great! The potato onion seeds from one vendor that I planted had a 20% germination rate.
Provide a strong light for 12 hours per day. Grow lights are great but not necessary. I use 40W florescent lights for all my seeds/seedlings and they flourish!
After the majority have sprouted, the lid or plastic wrap may be removed. Start watering from the bottom.
Fertilize with diluted fish emulsion or compost tea every couple weeks.
When the plants reach approximately three inches (7.5 cm), they can be hardened and transplanted outside. Ideally, they should have at least three leaves when transplanting. If it is too early to harden and transplant, and they grow taller than three inches, trim them down to two or three inches; it will not harm the plants to trim them. Keep trimming them down once or twice a week until it is time to harden and transplant. Trimming will help plants from becoming too top heavy and help keep nutrients in the roots and tiny bulb rather than the foliage. Trimmings can be used for salads, soups and cooking.
Hardening off is the process of adapting plants to the outside so they can adjust to sunlight, cool nights, and less frequent watering. Begin hardening in a sheltered location for a few hours on the first day, and increase an hour or two each day, until the seedlings are outside overnight. The entire hardening process should take 1-2 weeks. If you expect frost/freezing temperatures while they are outside, it is a good idea to cover them during the hardening period.
Ideally, seedlings should be transplanted when temperatures remain above 46°F (8°C) at night. Consistent exposure to temperatures below 50°F (10°C) for more than 10 days can cause onions to bolt rather than producing big bulbs. Onion seedlings can be planted in the ground 4-6 weeks before last frost. Having said that, although onions can tolerate cold and light frost, a hard freeze is brutal on seedlings, so protect them with mulch should temperatures plummet. I use leaves from the previous fall to cover them instead of wood mulch chips. Either way, remove the mulch as soon as your last frost date has passed.
After they have been hardened, select a location where they will be in full sun all day (at least six or more hours of direct sun), prepare the soil with compost or compost tea. Gently pull apart or separate any clusters of seedlings, taking care to break as few roots as possible. Plant individual onions about four inches (10 cm) apart by poking a shallow 1/2 inch to one inch (1-2.5 cm) deep hole in the soil and dropping the onion and roots in the hole. Cover with soil. If you cover more than a 1/4 (0.5 cm) inch of the stalk with soil, it is planted too deep - you want them to grow on top of the soil. Water gently.
Sowing Seeds Directly In the Ground
October 2022, I sowed 50 Green Mountain onion potato seeds directly in the ground. None of them survived the brutal Wisconsin winter; zero germinated. In a more mild climate, you may have better luck sowing seed directly in your garden.