Preserving fruits and vegetables has been a problem since antiquity. Warm climates typically have food available in some form year round. Winter food crops are possible in green houses as well. Lettuce and tomatoes, possibly spinach are common now in the winter by importation as well as growing in green houses.
Root crops are typically available late in the season, and the unharvested ground can be covered with VERY thick mulch to keep it from freezing. Often pine and spruce branches are added on top of the mulch to facilitate finding it..and to shade it out to avoid a freeze thaw cycle.
Kale grows in the winter, and where it does not grow, you may experiment with just how long they remain good under the snow.
Other than root cellars, that are discussed elsewhere, foods can be buried. Cabbages are typically buried for winter use in the north. Potatoes are stored in potato houses in the north for planting and winter use, but they may also be buried. The one drawback to burial, is that if you do not look them over regularly, one bad cabbage may infect dozens around it. This is also true in root cellars and other produce and meats.
Read up on these techniques on line...The resource is good, why not use it.
You should start buying alcohol now. Look for sales etc.. Buy rum, brandy and vodka by the cartload and store it underground. Just keep the covers from deteriorating.
Very high proof alcohol is best, but you can use brandy, high proof rum , cordials etc.
Preserve fruit in alcohol. This is common all over Europe, I do not know about elsewhere. Do not be prudish about alcohol. There are plenty of calories in Alcohol, and more in brandies, Rum and cordials from the sugar. You are not doing this to become a drunk...though this is a danger in depression causing circumstances. If you are afraid to consume a lot of alcohol preserved things, just cook the product before eating it. Evaporate the alcohol in the cooking process.
Some cultures, like mine in the Aeolian Islands, cook fruit and wine down to a syrup to preserve it and as a sweetener.
Rumtopf is a German method of preserving fruit throughout the season. You start with the first produce in the Spring, macerating the fruit in bite sized pieces(or whole small fruit pitted) in sugar. 1 pound of fruit to 1 cup of sugar is common. Then cover the fruit with rum. Now, you can cover the rum with plastic wrap, but they did this for centuries without. I believe you can do this without sugar in extreme conditions as long as the liquor is strong, but the product will not be as pleasant.
Add more layers of fruit as the season progresses. Each fruit must be in the alcohol for about 3 months.
About half way through the summer, you may add a flavoring like Cinnamon, Star Anise or Nutmeg...your choice. In the real world after the end of days, spices may not be available. In that case, there are tons of herbs that could be used, though they may not be appetizing to look at. Anise flavored herbs and seeds are abundant in herb gardens. Chervil, Sweet Cicely, Angelica,Fennel seed, thyme, and others might suit you. The first time you use them, make a small batch in a small container to see if you like them.
It is not necessary to add more sugar after the third month, unless a taste tells you it is necessary.
This three month period is long enough for the apples and late fruits to be mostly used up in the fall, and you will then need variety. Pumpkins and squash will last this long as well. Serve with meats, on desserts, on their own like pickles, cooked or raw. Brandy is commonly used, and I like that idea best. Cook those into fruitcakes and muffins! Remove heavy skins from fruit by blanching briefly in boiling water and skinning them in cold water.
Each addition of fruit should have a weight on top, like a plate that just fits inside the container, to hold the fruit under the surface.
So, in your post apocalyptic world, where do you get alcohol and sugar. Sometimes you have to make sacrifices to survive. Some fruits may not taste as good in maple syrup or sugar, but if you need the fruit to survive, you eat it anyway. Preserve nuts in maple sugar and syrup. Sugar beets are a good crop to have, and other beets and root vegetables can be used to make sugar...you just have to learn. Also, root crops may be very plentiful and easy to grow for this purpose and you may have too much for normal use. Honey would be usable, though again, you may not be happy with the flavor.
Making sugar is nothing more than grinding sweet things up, boiling to extract the juices and squeezing out the rest. Then boiling or evaporating the liquids till they crystallize, or use them as syrups. Grapes, and other sweet fruits will work, and wine that has produced even more sugar from fermenting will also boil down.
Make your own alcohol from grains of almost any kind, potatoes, wine, sugar syrup of any kind. Buy or make a still and watch out because blindness is a byproduct...I guess, of high alcohol content. But then they said that about masturbation too.
You cannot be prudish about alcohol. We are talking about survival here. However, you should know yourself. If you do not handle alcohol well, don't drink it. Do cook out the alcohol before consumption. THE BIGGEST THING YOU HAVE TO DO HERE, IS NOT TO MAKE THIS DECISION YOURSELF. YOU CANNOT KNOW HOW YOU HANDLE ALCOHOL. ASK YOUR FRIENDS AND FAMILY HOW YOU ACT UNDER THE INFLUENCE. YOU JUST DO NOT SEE YOURSELF WHEN YOU DRINK.
Now is the time to buy things like sugar, alcohol, containers etc.
Sugar can be bought in huge quantities, especially if you think that you will be going into this lock-down mode. Dampness will make it harden into a sold block, but it will still be usable. The big issue is water...it will dissolve, and ants...they love it. Keep the sugar sealed up completely, and high and dry.
Alcohol will last forever as well...Just evaporation and damage to the container will affect it. some alcohols will not be stable in taste, like wine and brandies that can be aged, but still the alcohol is alcohol. Seal it up, and if there is a cork, keep the bottle lying on its side. Alcohol sometimes evaporates over time.
Ceramic(stoneware) Crocks are great for storage and other tasks like Corning meat. Stainless steel is good, though even that can rust if improperly stored. Glass is perfect, but breakage is an issue....Are you careful? Of course the glass can be free, just collect it over time and store it away...wine jugs, demi-johns etc...glass candy jars and canister sets are ideal, but use wax or tape or fat to seal the top.
Fats(pure), unsalted.butter(best clarified), shortening, paraffin, meat fats that are carefully strained will all seal the surface of the preserves if plastic wrap is not available.
PRESERVING THIS WAY IS SOOOO EASY!
Making wine is another skill that is very wise to know. What do you do with fruits and some vegetables that are abundant and hard to store for long periods. Making wine increases storage time, and preserves the calories. Also, if the water may not be safe to drink, the wine will be. Wine, and beer(another good skill, though grain is a bit easier to store) were perrenial drinks in Europe becaause the water was very generally tainted. These provided safe drink. It is now an issue here that the water is less and less safe out in the wilds.
A by product of wine is vinegar. Be careful to make the wine well, or you will have a lot of vinegar.
Vinegar, though pretty acidic, can be used as a drink in an emegency, but it can also be used for pickling. Be careful though, because the acid can vary wildly and the preserving power will be affected.