The Boiling Water Method for Onion Seeds

I sowed onion seeds back in February. Three varieties: Carmen and Danilovsky in large cups. Only a single, pathetic little blade sprouted! Now I’m thinking about pre-soaking them in the paper towel/baggie method.

I sowed Yalta onions from Tumanov’s seed shop. The germination rate was 100%, and the little loops popped up in literally 5 days. Tomorrow I’m sowing White Yalta and Red Yalta, hoping for the same results.

I soaked my onion seeds overnight in a hydrogen peroxide solution (1 tbsp peroxide + 9 tbsp water, saw it on YouTube). Then I kept them in damp paper towels on a heat mat for two days. The germination rates varied wildly! My Neman variety sprouted like a forest, while Francisco had zero sprouts, and Daytona only had 1-2 seeds crack open. Seed viability is so unpredictable.

When sowing onion seeds, it is crucial to sow them on a perfectly flat surface. Press the seeds slightly into the dirt and cover them with an even layer of soil (no more than 1 cm / 0.4 inches deep). This creates identical conditions for all seeds and ensures uniform germination.

Proper watering is also key. I use the boiling water method: after flattening the soil in the container, water it with very hot water (around 70°C / 158°F). Wait until the soil cools down to about 50-55°C (120-130°F)—test it with the back of your hand; if it’s tolerable, you can sow. Pour the hot water carefully (I use a spoon) so it spreads evenly without digging holes in the dirt.

Sow the dry seeds on this hot seedbed, press them in, cover with dry soil, lightly tamp it down, and mist the top layer with a spray bottle. Cover with a plastic bag and place in a warm spot.

On Feb 17, I soaked onion seeds of various varieties (Exhibition, Candy, Barito + 3 others) in small plastic condiment/sauce cups. I kept them there until they started to crack open. I originally planned to carefully place each sprouted seed into paper towel rolls with soil, but that is way too time-consuming.

Instead, I took the easy route: I grabbed six 0.5L (16 oz) plastic cups, filled them with soil, and tamped the surface down flat using the bottom of another empty cup. I spread the pre-sprouted seeds on top as evenly as I could (wet seeds stick together, so it’s hard to separate them individually). Covered them with soil, tamped it down slightly again, and sealed the cups with plastic wrap.

By the third day, green sprouts were already showing! Huge thanks to Lyudmila for the tips.

I’m planning to sow onions tomorrow. Should I add vermiculite to the potting mix for onions, or is that unnecessary?

There is no need to add vermiculite to onion soil. Don’t do it.

What kind of containers do you use for onion seedlings? I want to cut a 5-liter plastic water jug, but I’m not sure how much height to leave. I thought you needed at least 15 cm (6 inches) of soil?

1-gallon plastic water jug is perfect! Cut it so the overall height of the container is about 7-8 inches, but you only need to fill it with about 3 inches of soil. That is more than enough for onion seedlings.

What is the best temperature for onion seedlings? If my greenhouse is around 15°C (59°F) during the day and drops to 12-13°C (53-55°F) at night, will they be okay out there?

Lyuba, those temperatures are absolutely perfect for onion seedlings! Once they germinate, they need to be moved to the coolest, brightest spot possible. If the greenhouse gets too hot on sunny days, just provide a little shade and mist them to increase humidity.

As for feeding: start fertilizing about 10 days after germination. Start with a weak solution (1g of fertilizer per 1 liter of water) and gradually increase it to 3g per liter. This year I bought a specialized “Agricola for Onions & Garlic” fertilizer, I’ll alternate it with urea.

Elena (El_ena38), now that your seeds have sprouted, you need to move them to the coolest possible spot right up against the window glass. Do not put them under grow lamps. You can place a plastic bag with holes over them to increase humidity. As they grow a bit taller, you can sprinkle a little dry soil around the base.

How long do they need to stay in that “coolest spot”?

Until their first “haircut” (trimming the tops).

Ladies, so the onion seedlings grow, I give them a “haircut” (trim the tops) down to the edge of the cup… but then they grow back. How many times do I trim them? And how do you plant them out? One by one? Sounds like a pain in the neck! Do you tie them to little sticks? And what if they flop over on the ground?

I just pictured someone tying onion blades to tiny sticks… 😂

Nelly, planting onion seedlings is super fast. You literally poke a hole in the dirt with your finger, shove the seedling in, and sweep the dirt back with that same finger. Last year I planted two 6-meter (20 ft) beds incredibly fast. It’s a bit of a squatting workout, but it goes quick. If they flop over, don’t worry about it. Just don’t let them dry out. The onions will figure out how to stand up on their own.

Here is my planting method: First, I moisten the soil in the container and carefully pull the block of seedlings out. If there is a massive root ball and it doesn’t fall apart, I actually snip off the bottom part of the roots (those are the old roots anyway). I also give the green tops one final trim so they aren’t longer than 18-20 cm (7-8 inches).

In the garden bed, I make three long trenches. I quickly lay the individual seedlings out along the trench at the proper spacing. Then, holding the seedling upright with one hand, I pull the dirt back over it with the other. It goes surprisingly fast!

I don’t plant the super thin, wimpy ones. I gather them in a bunch and bury the whole bunch at the edge of the bed. They gain strength quickly, and I use them as backup replacements if mole crickets eat any of my main plants. To avoid wimpy seedlings in the first place, you need to trim them frequently, feed them well, and keep them watered.

I was so stressed out planting onions last year. I later read that you are supposed to submerge the entire root ball in a bucket of water and pull the seedlings apart while they are underwater so they don’t tear. This year I’m just going to plant them in bunches anyway, since I just want them for white bunching onions.

I have a question about my harvested onions. Sometimes, underneath the dry outer skin, the next fleshy layer feels dense and incredibly sticky. Is this a disease? Also, sometimes there is black powdery mold on the inside wrappers. How do I treat this?

Elena, if the layer is sticky, that is a 100% guarantee of a Thrips infestation. You need to treat your crop with a serious systemic insecticide before planting next season (though you can’t eat the green tops if you do). Alternatively, you can heat-treat the onion sets in hot water before planting, but you have to peel the outer skins off first.

The black powder is a fungal mold disease. If you only have a few bulbs like that, it’s best just to discard them and not plant them.