You can try the black plastic bags to warm the soil, but I would NOT plant through them! They're meant to go on top of your beds a few weeks *before* your last frost date and be removed when you actually plant out your transplants. Remember, plastic isn't porous, so among other issues, if you leave the plastic there after you plant, no water can get to the roots.
The cloches are fine but they're really meant to protect young plants, as opposed to warming larger plants during the growing season. A general note about cloches--the glass ones are tricky since they don't ventilate, and with the cloche alone you have to be very vigilant about regulating the temperature. I personally prefer to cut the bottom off a 2 liter coke bottle and leave the cap off, as an individual cloche for a young plant.

If you would like to try to extend your growing season a little later into October, that can be done--but I would recommend constructing hoop houses or row covers. That will give you much better coverage for an entire row/raised bed, and can help prevent frost-nipped plants that you're not yet ready to surrender to the autumn weather. It might buy you a couple of weeks if you are lucky.

I am not sure why people say to not use black plastic or not to plant through it but my whole garden is covered with black plastic and it works wonders. If I lived where the temps were much higher I would'nt use it but in Mi., for me it works and I will continue to use it. Take a look at some of my pics and you can see the black plastic in my garden.

Here is a link that might be useful: garden pics

BP is a personal choice; some have very strong opinions for or against it. I used it for the first time last year, and loved it!

Pics are below, but here's some thoughts:

Warms soil- CHECK!
Prevents weeds- CHECK!
"Sour" soil underneath: nope, not here.
Water retention: CHECK!
Drainage (to prevent pooling): Poked a few extra holes in the pathways to allow drainage when it rained with a 3/4" aluminum pole. Worked like a charm.
Held down with either rocks, bricks, or my personal favorite: those 3-tier metal tomato cages cut down to rings with 'legs' that I poked down into the plastic.

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Here is a link that might be useful: {{gwi:32131}}

I LOVE my black plastic. I buy it in 20 x 100 foot rolls at Home Depot or Lowe's. To cover all of my planting beds, it takes about 12 full rolls. Luckily, it lasts about 3 years -- good thing, since the price for that size roll has increased to about $80 the past few years as oil prices spiked.

As state above, it warms the soil, prevents almost all weeds (a few do get through, especially in the 2nd and 3rd years as you get more holes), traps moisture, etc.

I take a drill with about a quarter inch bit, and drill holes all around the roll right through the cardboard box, spacing the holes about 3 to 4 inches apart. This five minute job results in the plastic being studded with thousands of little holes, big enough for good drainage but small enough to prevent weeds from growing through.

IMO, black plastic garbage bags are too flimsy, and probably a lot more expensive per square foot than 6 mil black plastic.

Black plastic mulch doesn't need to be thick to be effective. Actually clear plastic would warm the soil better but then you would have no weed control. I lay a few miles of 4' rolled black plastic every year by tractor pulled plastic layer and wouldn't plant most transplant crops without it. However I would not plant seeds and cover with plastic as mentioned. If you are not transplanting through it cut a "X" slit with a knife and tuck the cut corners under, plant your seed and cover over edges with a little soil to keep wind out and prevent the plastic from smothering your seedlings.

If you have a larger area covered you can either poke holes in the plastic or put drip tape under for water supply to plants. The problem with even small holes is that grass will find the light and before long you may have defeated your purpose of weed control.

If you are serious about season extension consider also using a floating row cover over newly planted crops that are set out a few weeks earlier than last expected frost date. You get the protection of the cloches without the potential hazzard cooking your plants if the sun gets to them before you do.

Well I don't know how to make floating row covers nor do I know what they look like. Also I'm on a very limited budget so I don't think it's an option to make them at this point and time.

Regarding the cloches, I plan to use the 2 liter pop bottles for my seedlings. They're mainly going to be used to protect my seedlings from windy weather; the fact that the cloches act as a mini greenhouse is just a bonus. Should I put holes in the cloches using a drill bit? If so, how many holes per cloche and what would the size of the holes need to be (1/4", 1/2", etc.) I'd like the holes to be big enough that my seedlings won't be overheated but small enough that windy weather won't knock my seedlings down.

The normal rules of gardening don't apply to me, because
I'll be growing things in containers. I'm guessing that the soil will boost the temperature? My main concern is at night; the temperatures during the day are warm enough
that it's not a big issue. However they drop down
to about the mid 40's in the evening.

Where Can I find black plastic that is used for gardening?
How much would I need? I realize this depends on how many containers I have and what the dimensions of the containers are, but I have the dimensions for the containers I have.

Here are the dimensions for each container below:

10 gallon (have 4 of these)

Length-19"
Width-13
Depth-12"

15 gallon (have one of these)

Length:23"
Width:17"
Depth:15"

I have a container that is roughly 10 gallons, but slightly smaller so here are the dimensions for it:

Length:19"
Width:14 and a half inches (it will not let me put the back slash so that is why I had to spell it out)
Depth:11"

The other containers I have are a 5 gallon circular container and pots that have rims with a diameter of 9" and 10". I don't know the dimensions for these though.

I cannot afford to spend $80 on the black gardening plastic. Can you recommend a place that sells it cheaper? Over the internet is fine, so long as I don't get ripped off with shipping and handling charges.

K-g:

Most any hardware store or general merchandise store (Meijer, K-mart, Home Depot) will sell 10 x 25 foot rolls of 6 mil black plastic for around $12 to $14. Look in the paint department -- its used for dropcloths, among other things.

Regarding clear plastic -- as the one poster above said, it does NOT keep down the weeds. I put a big section of clear down years ago, when I was first doing this. I figured weeds would grow under it, but it would keep them down. WRONG -- they pushed it up 2 to 3 feet off the ground in some spots. Even though they were half cooked, the weeds just kept growing -- nasty things like pigweeds and sow thistles.

I'd use the heavy duty trash bags as you originally asked. Nothing wrong with that.

I use trash bags too...works fine. One thing I'm doing this year is poking holes occasionally along the plastic for moisture, as there was no moisture underneath when I removed it. But, the area around the plants were fine, had a good crop.

Soil in containers warms up much faster and can get too hot much faster than in-ground beds. So much of the above advice may not apply to you growing in containers. 40 degrees poses no problem for most things as the soil will be warmer than the air. And with plastic containers you will lose some heat out the sides of the containers too, not just the top.

So while you might want to use the plastic trash bags initially you'll need to watch the soil temperatures closely to prevent plant damage once the summer heat arrives.

Dave

There's also olive mulch which warms the soil better than black, but still doesn't let the weeds grow.

Here is a link that might be useful: Solar mulch, olive colored

I have black trash bags cut and covering the soil now (I had some left from last year) and I also used black landscaping paper.

My only intent is to heat up the garden faster than the rest of the year prior to me planting.

I'm in zone 5 so any extra heat I can get in the ground earlier would help.

Well in regards to using the black bags or the black roll people are referring to, I'd be using primarily for warm weather vegetables (zucchini, cucumbers, butternut squash, maybe okra, maybe eggplant, green beans, tomatoes, and watermelon).

I realize that cool weather veggies like cool weather but I'm concerned about night time temperatures for cool weather vegetables; that the temperature will drop down too low even for cool weather vegetables.

Another concern is that it will slow germination which will defeat the purpose of growing my cool weather vegetables early.

2 litre bottles with the cap side up, then unscrew to ventilate