Onions - Growing from Seed - Using heat mats and shallow pots

Note: This article explains how to start onion seeds using a heat mat and shallow pots. To read about how to start seeds in deep pots, without heat mats, CLICK HERE).

There are literally hundreds of varieties of onions grown in this world, but unless you grow your own you usually have access to only a handful of varieties from the grocery store.  If you rely on growing onions from ‘sets’, (the little onions, about the size of a large grape) your options are often still quite limited.  Growing from 'sets' has other disadvantages too; often they will produce a significant number of ‘doubles’ (meaning smaller onions at harvest-time) or they bolt and go to seed, which makes them tough and unpalatable.

   

Here is a guide you can follow that will increase your chances of success at growing onions from seed.

Onions going to seed.

Here in the Willamette Valley of Oregon (USDA Zone 7b - Link to finding what zone you are in), we do best to grow what are called ‘long day’ varieties which include Cipollini (chip-o-leenie) both red and yellow varieties, Southport White Globe,  or White and Yellow Sweet Spanish. (Choosing Onions-Long or Short Day?) There are many more varieties to choose from; check your seed catalogs or go online to see a greater selection. (Be aware that, if you wish to save your own seed, you must choose Heirloom/Open Pollinated non-hybrid seeds. Those listed above are all Heirloom varieties.)

You will need:

• Sifted potting mix
• Sand (optional)
• Pots: We used to use rather large pots (4" x 6" deep - 25 seeds per pot) but have shifted to using tofu containers with holes drilled in the bottom (see picture below), or jumbo six-packs. These use less soil for the same amount of seeds.
  
• Seeds: start with fresh seeds each year; onion seeds lose viability within one or two years. If you wish to save seed, choose Heirloom/Open Pollinated varieties. Choose a variety appropriate to the length of your summer days.
  
• Greenhouse or grow-lights
• Heat mat (seeds will germinate better with a little bottom heat).
  
• Plant-mister  

When to start: About eight-to-ten weeks from the time you wish to transplant them into your garden. Usually we get the seeds started around the middle of February but, if we have extra seed, sometimes we experiment with starting a little earlier (this would only work using a heat mat for bottom heat). By late April the seedlings will be ready to set out into the garden.  They are ready when you can see a clearly defined "bulb", 1/8" to 1/4" pushing above the soil-surface in the pot. At this stage, the sooner they get into the ground the larger your onion bulbs will be at harvest time.  It’s always a good idea to have a bed in mind that you’ve prepared during the previous fall since it’s difficult to prepare beds in the spring if you have a long rainy season like we often experience here in Oregon. The seedlings can handle a touch of frost at this stage but its no fun transplanting them in really muddy garden beds. Some people wait until early or mid-March to start seeds and still have plenty of time for the onions to ripen.

 

Another option, if you have raised beds in your grow-tunnel/greenhouse, is to transplant the 'starts' there - instead of outside). Onions are easy to use to fill in spaces between larger plants, maximizing your raised-bed use. But they need full sun so don't plant them where they'll get a lot of shade later in the season as other plants mature.

 

When planning ahead for planting, consider also the practice of companion planting as some plants are more or less compatible with onions (LINK).

Pots: If you will need to wait more than 8-10 weeks, to transplant the seedlings into their permanent growing space, follow this guide for planting in 4"x 6" pots. This way you won't run the risk of the onions becoming root-bound before you transplant them.

 

In recent years, because we have increased space in the raised beds of our greenhouses, we've shifted to using re-purposed tofu-containers with holes drilled in the bottom - (see picture below - approx. 3"x 5"x 2" deep) (we eat a lot of tofu! - if you're local to us and need some of these containers for seed-starting, let us know!). Jumbo six-packs work too.

Tofu containers for seed-starting. We've used them as 'six-packs' too. Though the roots of the 'starts' grow together, it's easy to tear them apart without damaging them. The tofu containers, 3"x 5"x 2" deep, use less soil than jumbo six-packs so we save on potting mix.

Soil/Sand: Start with a good organic seed-starting mix.  It doesn’t need to be a premium potting blend, in fact, if you start with too rich of a soil blend you can experience a condition referred to as ‘damping off’ which looks like algae or mold growing on the surface and which causes the young seedlings to rot as they emerge from the soil.  One way to help eliminate this condition is to sprinkle a thin layer of sand over the seeds.  By keeping the soil damp but not too wet and having good ventilation you shouldn’t have this problem. 

Onion seeds can be started in a variety of containers.

Fill the pots with a sifted soil to about 1/2" from the top (tiny seeds find it difficult to germinate in soil with large chunks of material). Level out the soil, tamping it down with the bottom of another pot to create a level surface so that all of the seeds will be sewn at the same depth (otherwise they germinate unevenly). Label each pot with the name of the variety and the date you started them. 

Seeds: You may want to actually count out the seeds the first time so that you can have an idea of what 25 - 35 seeds looks like because that is about how many would be optimal to sew in the tofu-containers or six-packs.  You can adjust up or down depending on the size container you choose.  The idea is to not have an overcrowded condition that would produce weak and unhealthy seedlings.  Place the seeds in the palm of your hand and pinch out a few at a time. Gently drop them on the surface of the soil, distributing them as evenly as you can without becoming too concerned about accuracy.  It’s OK if some seeds are touching each other. Sprinkle a sifted layer of starting mix or sand over the seeds at a depth of about an eighth to a quarter inch.  Tamp it down again and water gently (a planter mister works great at this stage). 

Watering:  Keep the soil moist using either a small spray bottle or water them from below by putting water in trays and setting pots in them (light watering minimizes 'damping off' too). Tiny seeds, until established can be washed away with more aggressive watering techniques. Do not over-water! Soil should be damp but seedlings should never be in standing water. 

 

Heat mat: A heat mat is helpful for germinating seeds if you are doing so in an unheated greenhouse and nights still are getting cold. Heat mats can cause soil to dry out more quickly than ambient air so cover the seedlings with something to keep warmth and moisture contained.

 

Then it’s time to be patient, and let Nature do her work.

The seeds will not require sunlight until they have emerged from the soil, usually about two weeks from the time they are sewn, so you can keep them indoors where they will not freeze, on a window sill or in a greenhouse if you have one. Once the greens are up, they will require full sun. If you don't have grow-lights or a greenhouse, be sure to bring them inside at night if it looks like you may have freezing temps.

 

You will know it is time to transplant when the greens are stout and well-established and you can see a tiny bulb pushing up from the surface of the soil.

Teasing onion-roots apart before trimming.

Transplanting: Each pot of seedlings must be teased apart. You will need to trim back both the tops and the roots before trying to stick these tiny seedlings into the ground. You trim the roots so they're easier to slip into the holes and you trim the tops so that the pruned roots can support the greens above. Don't be too aggressive with your pruning however or it can shock the plants and make it hard to recover.

First, dump the whole pot into the palm of your hand.  Next, separate the clump into several sections (maybe 10-12 seedlings in each clump).  Hold one clump by its 'greens' and gently tap the root ball until most of the soil has fallen away.  Tease the seedlings apart and lay them back in your hand so that the small bulbs are in a line (see picture below). Using a scissors or hand pruner, cut away the tips of the roots, so they'll easily slip into the soil.  Trim the tops to about the same length as the roots. Now set the clump into another shallow container with a little water in the bottom to keep them from drying out while you prepare the rest for transplanting.  Prepare only as many as you are able to transplant in one session. If you end up with extras, surround their roots with some moist soil and keep them in the shade until you have time to get them in the ground.

Trim roots and greens to same length.

If you are going to plant them in rows, open up small holes about 4-5 inches apart. When we plant our onions this way, our beds are 2'-3' wide, with several rows in each bed. 

 

In recent years we have planted rows on the south edges of the raised beds in our greenhouses and in and amongst other crops that won't shade the onions later in the season (for example, on the south side of tomatoes or peppers), or we plant them among crops that will be harvested before the onions are mature, leaving the onions in full sun (like among lettuce plants).

Make holes: To make the holes you can fashion a planting stick called a ‘dibble’ from a smooth branch or a ¾ in dowel with a point or, just use a ‘Sharpie pen’, or even your finger to make the hole.  Be systematic: make  5-10 holes and then go back and drop a single onion in each hole.  Gently press the roots into the hole and pinch the soil around each one, making sure the part that was under the soil in the pot is covered when you transplant, leaving only the green top and a tiny bit of the bulb showing.  Be sure that all the roots go in the hole and don't "J-root": with bits of the roots poking out from the soil; this will dry out the plant and force it to use extra energy in turning its roots back down into the soil. You’ll get the hang of it after a few tries and be able to transplant hundreds in no time at all!

Onions in a wide bed.

Now the focus becomes keeping the bed weed-free and well watered.  Mulching between the onions with fresh grass-clippings keeps moisture in the soil and fertilizes the plants each time you water through the clippings. Just don't bury the seedlings! Once the plants have become established and the warmer, sunny weather settles in you’ll be amazed at how fast everything grows.  Feel free to thin out your onions when they are immature and be sure to use the whole thing, greens and all. 

If you have planted non-hybrid seeds, hold back a few onions to replant next spring to save seed. Onions are biennial meaning they don't produce seed till their second year. By collecting your own seeds you can begin the process all over again, and saving seeds, dear friends, is one big step toward greater food security!

Link to another post we wrote about onion-growing.