Winter Seed Sowing: Pros and Cons

Somehow, miraculously (and thankfully) seed starting season is just about here once again. Perhaps in no other year (or years?) has thinking about starting a garden ever been more essential.

I’ve been gardening since I was a kid, yet if there is one thing that I keep trying to improve my skills with is with germinating seeds yet I am still learning. You’d think that now that I’m 60 that I would have mastered all of the tricks and tips of the trade, but the truth is that it’s never quite that simple. What I have learned is that research, patience and knowledge go a long way when it comes to success with seed starting.

I think many of us started our gardening lives by sowing a seed, and I do believe that it’s one of the best ways to inspire a new gardener about the wonders of gardening, especially with children. But if there was one thing that I could change about my gardening career, it would be to pay more attention to the details when it comes to seed sowing.

In my 20s I remember spending hours and hours reading and dog-earing the Thompson & Morgan seed catalog, which at the time was the catalog, at least here in America, where one could find endless varieties of annuals, perennials and even bonsai seeds (I know. Hey, I was young.). I tired and failed countless times, but I also succeeded (obviously by accident) with some. I remember how thrilled I was when a packet of Cyclamen seed produced one seedling.

Today, at least I know much better. And, things are so much easier with the great research tool that we all have – the internet. On the other hand, things can be so frustrating with the wrong information and as you probably know, the internet isn’t always right. Factor in social media, and things can get downright crazy. I saw a woman sowing pepper seeds into milk jugs the other day and setting them out in the snow.

Im not angry here, just passing on a warning. Be careful when following advice when it comes to gardening found online. With seed sowing, right not the trend is something called Winter Sowing and I wanted to touch on the concept and try to explain the pros and cons and to help some of you to understand that waiting until spring isn’t the worst thing to do when it comes to starting your seeds. Mostly though, I would like you all to start thinking about what seeds need to be sown when, and why.

My advice is to first look deeper into who you are following the advice of, for often the influencer lives in a milder climate such as Georgia or North Carolina. If they are sowing seeds that cannot withstand a hard freeze, thaw, and then freeze again cycle, and you are sowing the same seeds in your New York State garden (or milk jugs out on a deck in deep snow), just triple check if you are timing everything correctly. Never assume that every perennial requires a winter freeze to stratify their seeds, or that a ‘cold-weather’ crop is actually a cold weather crop. Remember, cabbage and broccoli may thrive in cool conditions while mature, especially in the autumn, but they germinate best in warm conditions. The same goes for most poppies, by the way.

Winter germination projects work best with those plants that really need a long rest in cold, moist conditions. Primula, alpines, trees and shrubs for example. Actually, most primula (primroses) don’t need a chilling, and as for the old myth that bread seed poppies must be sown on the surface of snow, a myth is a myth. All poppies can withstand freezing temps for a bit, but their seedlings cannot. Besides, poppies such as Shirley Poppies and Opium poppies really are best direct sown (in early spring is fine) and thinned to leave one seedling per square foot. The seeds will survive being sprinkled on snow, but that romantic notion does little to enhance their progress. The seeds still won’t germinate until those first 70º F days in spring.

As for other plants, I do practice winter sowing (and autumn sowing). There are dozens of seed trays outside here right now waiting for spring to get them going, but most are woodland plants like Anemonopsis and Disporum. I actually read a post on a blog where a young gardener was boasting about sowing her zinnias, peppers and onions out in her milk jugs set into the snow in January. This may be fine for onions, but still – they won’t grow well, if at all, and more often than not, the seeds are sown too thickly, especially with her broccoli. I do sow all of my brassica’s in containers outdoors, as well as all of my zinnias, marigolds and rudbeckia -except I sow them in late May, as then the sun is brighter and the day length long enough to not cause the marigolds to bloom too early, and the zinnias – who need total warmth, can grow quickly with a minimum of root disturbance. I should also mention that I sow them all into individual cells, also to minimize root disturbance.

1.Don’t assume that a ‘cool-weather’ crop wants cold weather to germinate in. Such a common mistake, but many of us make this. Parsley plants can withstand freezing temperatures (which is why they are often sold along side pansies in spring) but this exposure, if too early and if the plants are larger, will cause parsley (which is a biennial) to bolt by early summer, bloom and then die. Celery has a similar problem. These are crops that should never be exposed to low dips into the 40s and thus should be set out about the same time that you plant tomatoes. Artichoke seedlings do need exposure to cold weather for a week or two, because this is a plant that does want to be tricked into believing that it has survived a mini-winter, which will cause it to bloom.Just remember, cold temperatures send messages to plants, as does daylength.

Few of us benifit by starting something too early and most of us would be shocked at how quickly a seed sown in May will grow. Of course, there are certail crops that must be sown early but it all depends on your climate. Most cool-weather crops are best at surviving a long, cool autumn than the variables of spring. A safe bet for early sowing outdoors is lettuce, spinach, peas, green onions, radishes and arugula. All can be sown as soon as the ground thaws.

All brasica’s however while cold tolerant, prefer to germinate hot (near 90° F) and while seedling that have been hardened off can withstand light frosts, new research indicates that a later sowin is better if only to avoid the first hatch of the cabbage root fly maggot, so why rush things? Brussels Sprouts are always better if sown later in mid-June anyway, as day length will factor into when they want to start sprouts in fall. Never buy seedlings at a nursery in May.

2. Know if the seed you are sowing can be frozen. According to the Seed Biotechnology Center at the University of California Davis, not all seeds can be frozen. Do a bit of research on-line to first see if the seeds you are sowing do indeed need to be frozen before they bloom.

3. Know your Local Climate, and then know where your sowing inspiration lives and gardens. I love the blog The Reid Homestead written by homesteader Tamara Reid. Many of you may follow her too. An avid and experienced gardeners, Tamara often posts both about her seed germination adventures but also winter sowing. I can see from the comments that many enjoy the photos of her winter sown veggies and flowers, but Tamara lives in Washington State, smack in the middle of mild gardening zone 8b. If you live in New England, New York or Ohio, (Zone 5 or 5) following the advice without adjusting the dates might actually mean that you should be sowing the same varieties in March or April (essentially, Spring sowing in jugs outdoors. This outdoor sowing in spring is perfectly fine and actually highly recommended for many in zones 5 and 6. I encourage it, but again, it is spring sowing, right? It just doesn’t sound as provocative as winter sowing.

4. Consider Spring Sowing Outdoors Instead. The benefits of sowing outdoors slightly early are many, particularly when it comes to the quality of the light which just gets better and better as spring progresses. If I wanted to keep this post short, I could, (and should) just advise you to ‘Sow outdoors in late spring to early summer’ for most plants. It’s what I do, even though I have a greenhouse and a grow light system. The truth is, I sow in all locations, but it depends on the plant I am growing. Some seeds are started in the greenhouse in January (Parsley, artichokes, celery) some, in February (sweet pea flowers, many hardy annuals, poppies) others I must start in February under lights with warm temperatures (impatient, begonias). Some flats are chilled out in the snow, sown in December or January but get moved into the greenhouse in March to get them going. This includes larkspurs, some perennials, some primroses. Others are just sown in March in the greenhouse, some with bottom heat (Cabbage, broccoli, Kale), others without (like plugs lettuce and arugula).

5. If you are practicing winter (spring) sowing, sow very thinly and sparingly. It’s common to see success stories around a crop of seeds that were germinated in a jug, but that were sown so thickly that one would need to tear apart the root ball to separate each seedling. I rarely see any well-grown broccoli , kale or cabbage seedlings, for example as they are often sown too thickly. I would suggest 6-10 seeds per jug (really) leaving about 2 inches between each seed if you want to raise the healthiest plants. Professional (and many home) gardeners often use plug trays which are best for most plants but especially those that resent root disturbance, and one can assume that most plants resent root disturbance, but particularly anything in the carrot family, but also cabbage and kale. Plugs are difficult to set outdoors in late winter or spring because of the smaller cell of soil mass which can freeze too easily, or dry out if the tray is covered. Larger cells (1.5″ – 2″) are better but I still tend to only use these for spring sowing outdoors only if I can water them every day. They are much better suited for a cold frame or a cool greenhouse (or under lights).

6. Don’t try to be a generalist-Sower. Again, never assume that all seeds want or need to experience cold germination – even if you see someone sowing broccoli and zinnias in the jubs set out in the snow, proceed carefully and knowledgably. The biggest mistake one can make is to just open random packets of all the seeds they want to grow and then go sow them early in milk jugs set outdoors. Know why you are sowing, and why they might require a freeze.

Also, know the difference between ‘cold hardy annuals’, ‘hardy annuals’ and tender annuals, (don’t worry, most of us really never knew the definitions, but for a good reason – these are terms created in England where winters are mild.). These English terms that were used a century ago by British gardeners may work for gardeners in Portland, Oregon and Raleigh, N.C., but they are rather misleading for someone in Chicago or Boston where winters are unpredictable and often cold. Sure some ‘hardy-annuals’ are self-sowers, but others are not. Poppies may self sow if they are opium poppies, but rarely do shirley poppies self sow in zones 5 or lower. Opium poppies often germinate in late summer which delights us with the hope of field of poppies, but if winters are harsh, the seedlings won’t survive. It’s the seeds that fell into the soil that didn’t germinate that are the ones that grow in spring, at least in Zones 6 and lower. As you can see, the plants are a bit smarter than we are.

In the end…

Just as with news, just be sure that you are double checking the facts and where your advice is coming from. Winter germination is great if you sow in March or APril in the north (but then it’s sort-of like spring germination, right?). Outdoor germination in containers is even better in late April, May and June, at least here in the north. Know that some seeds must be sown early in late January or February, either in a greenhouse, or underlights, but again, know what you are sowing and do a bit of research and fact-check that research a second time. I only say this because It isn’t difficult at all for me to find completely contradicting advice online regarding the needs of a plant. Welcome to our digital world where anyone can be an expert.

As an example, I thought that I would research if zinnias could indeed benefit from a winter germination project (I saw a few posts with folks showing photos of their zinnia seeds out in their milk jugs set into the snow).

First let’s look at the genus Zinnia, while most of us simply refer to zinnias as one type of cut flower, there are over 30 species, and at least 5 and a few interspecific (crosses between two different species) that are commonly grown as bedding plants and cut flowers. Some of these species require different daylengths for germination (which hey, I did’nt know), and this affects when they bloom. As a side note: The same goes for the common marigold by the way, most of which actually grow better later in the summer as they wont bloom until day length is reducing to around 10 hours a day and many serious gardeners wait to sow their marigolds until June outdoors (I do, and the results are fantastic and fast). I was also interested in the fact that many commercial seed growers do the same thing.

If you are ever in doubt, referring to professional grower grow sheets (downloadable PDFs often available from the biggest seed companies) or by researching scientific botanical journals that offer specifics about each species, particularly where they are native and then calculating yourself using the elevation and climate of the type species to determine yourself what a plant requires to grow optimally. I know – geeky, but again – horticulture IS science. Just remember that these grow sheets are created for professional growers targeting specific dates to market. At the very least, it’s interesting (if not horifying) to see how many chemical plant growth regulator treatments most annuals recieve before reaching the shelves and our gardens. No wonder my cosmos and snapdragons are never as tall as I remember them being.

So, do I winter sow?

Sure I do. I parents did as well as my grandparents. Look, winter sowing is not new regardless of what some groups of Facebook claim. What’s new is the novelty of it all. Just sow smart, be informed and learn from any mistakes you might make without getting discouraged. It’s generally how all gardeners grow.

Pick up any American Rock Garden Society Quarterly from the 1930’s and you’ll see detailed step-by-step methods for sowing alpine or primrose seed in December, for example. In fact, most alpine gardening societies offer their rare seed exchanged in late autumn just so enthusiasts can sow their seed in December or January. Many woodland or ephemeral plants must have their seed sown fresh (cyclamen and corydalis) which typically means collecting seed in late spring or early summer to sow. Often these seeds won’t germinate until autumn or the following spring, some will require a winter or even two years of freezing temperatures before they will germinate.

Also, ask yourself: Why am I sowing early?

Many who promote winter sowing list the benefits. These range from “it gets me out into the garden earlier” to “It saves me money because I don’t have to buy expensive heating mats and grow light units.” Naturally, a milk jug is a bit like a poormans greenhouse as well. Most of the crops folks are sowing seem to be what they often consider as ‘cold-weather crops” like brassicas, lettuces and greens, and root crops such as beets.

IF YOU REALLY WANT TO TRY WINTER GERMINATION, TRY THIS:

Sow some cold-tolerant annuals or hardy annuals.

Just know that all ‘hardy annuals’ don’t fall into the same bucket. It all depends on your local climate. I’ve also seen it written that “a good sign that an annual is fine for winter sowing in jugs is if it self seeds or if the seed packets have phrases on it such as “self-sowing’, requires freezing, or “cold-tolerant'”. This isn’t necessarily true. A Shirley poppy may drop seeds that wont survive a frigid winter, but a larkspur seed will. It’s just not as simple as this. Also, even today I read in a book that one could sow poppy seeds in autumn, but I had to check and discovered that the author lived in England.

Horticulturally speaking, if a plant self-seeds, or prefers to, it’s generally safe to assume that it also means that it is a plant that dislikes root disturbance. Now, you may have read in some of my posts that I often recommend starting these fussy-transplanters in cells or plugs, and this is true, but always proceed carefully and time things perfectly. Never allow the roots to emerge from the bottom of the plug, or sow just 1 or 2 poppy seeds in a 4 inch pot and transplant promptly.

In the wild, these plants may be biennial in nature such as with fenner or Ammi majus or they may produce some sort of radical or tap root first that wants to tunnel straight down as soon as the seed germinates. My point is rules can be stretched or broken, but do so with knowledge.

SO in closing, as you can see the plants I choose to winter sow are rarely my vegetables or flower.In the end it all much more like spring sowing. I do winter sow primroses, some perennial seed but only to expose the trays to cold, snowy weather for a month or two. And, there are plenty of all plants that I sow in the greenhouse or under lights because they need warmth. They come into the greenhouse or under lights by the end of February so that I can get some growth on them before the summer heat. 

I also summer sow many biennials in flats outdoors, especially foxgloves, dianthus and pansies/violas. These are sown in August, and the trays kept in a cold frame or in the coldest part of the greenhouse until mid-spring.