You too can harvest your own veggies year-round even in a cold climate. Seriously, it's pretty easy. You just need to be willing to experiment with a few dollars worth of seeds (start with mini-packets of seeds). There is definite trial and error involved and you will suffer crop-loss at some point (welcome to farming). But you will also be rewarded with fresh, delicious food most of the time and most of the year. I focus on cold tolerant/hardy plant seeds on this page but be aware that in warmer climates and from April to October in most climates you will need to switch to heat/sun tolerant plant seeds. First thing to do is to examine the USDA Plant Hardiness Zone Map and figure out which hardiness zone represents where you live. Next, if you are in zones 1-5 (and probably zone 6 as well) you need to understand that the plants really are not going to grow much (or at all) during the coldest couple of months so you need to plan accordingly. That is, if you want to be harvesting in January you will need to plant seeds between August and October, let things grow to decent sizes hopefully through the end of November or so, and then maintain the plants (mostly by keeping them out of the elements) until you want to harvest. OK, now decide what scale of project you would like to try and then check out the resources, links, and recommended seeds to start with I've compiled below. Please Note: I'm continually trying to update this page so check back for updates.
Farming Scale -> Hoophouse / Passive Solar Greenhouse
Resources / References
You could of course try seeds for any of the crops mentioned on my website and in the above resources. For your reference, the MSU Student Organic Farm, Four Season Farm, and my Brines Farm are all located in plant hardiness zone 5 while Steve More is located in zone 6. In general, if you are in zone 6 or warmer zones, I say you should feel comfortable trying for example any of the following seeds from Johnny's Selected Seeds: Greens, Lettuce (that performs well in greenhouses), Spinach, Kale & Collards, Beets (particularly for their greens), Swiss Chard, and some Carrots, Onions, Leeks, and Turnips. Some herbs you could try are chives, cilantro, and parsley. If you are in plant hardiness zone 5 or colder zones you will want to pay a little more attention to the seeds you use. Note that with the preceding Johnny's Selected Seeds links you can sometimes refine your search for "cold tolerant" - look for a link on the right side of the page. I'm all about experimentation but if you are looking for seeds other people have had proven success with in cold climates (bearing in mind that winter weather varies from year to year affecting results) pay close attention to the seeds some of us mention in the above references. My short list is: Mache/Vit/Corn Salad, Claytonia (Miner's Salad), Arugula, Minutina, Kyona Mizuna, Mibuna, Tatsoi, Red Giant mustard green, Red Russian Kale, Space Spinach, Dark Lollo Rossa lettuce, Rouge D'Hiver lettuce, Winter Density lettuce, Hakurei Turnips, and Tadorna Leeks.
Backyard Scale -> Small Hoophouse / Greenhouse
Resources / References
Temperatures inside a smaller backyard scale greenhouse tend to be a little cooler than those attained in a larger scale greenhouse (largely because you have smaller square footage of thermal mass, namely the soil/ground, that is retaining heat) so I recommend first experimenting with the more cold tolerant seeds of the ones mentioned previously. Of my short list above I would sub-set it to: Mache/Vit/Corn Salad, Claytonia (Miner's Salad), Arugula (switch to Sylvetta Arugula), Minutina, Tatsoi, Red Giant mustard greens, Red Russian Kale, Space Spinach, and Tadorna Leeks.
Small Scale -> Cold Frame or Balcony or City Rooftop
Resources / References
Of my short list above I would sub-set it again to: Mache/Vit/Corn Salad, Claytonia (Miner's Salad), Arugula (switch to Sylvetta Arugula ), Minutina, Tatsoi, Red Giant mustard greens, Red Russian Kale, Space Spinach, and Tadorna Leeks.
Window Sill or Potted House Plant Scale
Resources / References
Check out Johnny's Selected Seeds Micro Mix and Sprouts selections.
Notes in General
Soil of course is one of the most important components. For all settings I suggest a healthy compost soil. I'm generally not a fan of potting soil mixes or conventional starter mixes, unless you make yourself. It's always best to know exactly what's in your soil. Where to get compost? You can always make it. You could use vermiculture in a more urban setting. Find a local gardening or 4-H club. Look for your local state university county extension agent or office. Even NYC has one. Check with landscaping companies and find out where they buy their compost if they don't sell any themselves. In an urban setting, another idea is to ask a farmer you like at your local farmers market if they might sell you some healthy compost soil (perhaps in heavy-duty garbage bags or 5 gallon buckets with lids). Of course, keep in mind that they will effectively be doing you a favor because it is one of their most important resources so be nice when you ask but I bet you could find some willing as long as they have room to transport it when they come to market. Farmers market farmers are typically pretty nice. Heck, I take some of my regulars compostable food scraps to compost on the farm.
The question I get asked the most is "What about watering during the winter?" and the short answer is "you don't do a lot of it, at all." This is particularly true in colder climates like zone 5 or colder. Remember, if you doing winter harvesting in zones like this, your plants really should have gotten most of their growth done by the end of November, so you are mainly just trying to maintain them. This does not necessitate a lot of water generally but will depend on your setting and how much sun exposure you get. So you really have to get a feel for winter watering, pay attention to the weather, and make the call since most settings are unique. In general, I don't water if there is no sun and the air temperature around the plants is freezing or colder. So basically I wait for days that will be quite sunny, I water early so it has time to soak in, and I make sure I cover the plants back up (plastic or row cover or shutting the cold frame) if temperatures are falling later after watering. If I see there will be multiple days above freezing day and night, I generally water a bit as well regardless of sunshine. As an example, here in cold cloudy Michigan I can go with only half a dozen watering days or so during the cloudiest/coldest couple of months. As the length of day begins increasing in February and we get more sunny days through March and April we water a whole lot more.
Good luck. Please feel free to email me stories and good links and resources online that I should share.