Sorry but are we talking about row covers - the white spun polyester covers that are used on top of plants to block insects and provide some additional plant protection against frosts? Or mulch, specifically black plastic mulch (aka landscaping fabric, weed barrier) which is spread over the ground to warm the soil?

From your questions I am assuming you mean weed barrier mulch not row covers.

Black plastic mulch, placed in tight close soil contact, will warm the soil in the spring approx. 10-15 degrees in approx. 10 days to 2 weeks depending on the amount of sun exposure.

As the late spring and summer come on it can heat the soil a great deal more. Sometimes too much in the warmer garden zones unless covered with a layer of some organic mulch to lessen the sun exposure. In zone 7 it is a great spring tool but usually needs to be removed or covered well in the summer to avoid cooking the roots.

Normally it is not removed before planting. Small X's or holes are cut into it for planting and it is left in place.

Hope this helps.

Dave

Plastic weed barriers for warming soil

Row covers

I think they are talking about row covers.

It depends on the weight you get. I use the standard weight. For anything planted that doesn't hate heat I use black plastic mulch also from Farmtek. So no weeds. I usually do not put the row cover on before planting but I would do that for clear plastic type things or Wall o Waters etc. I don't use row cover to extend much in spring really. I use it more for animal and bug protection. For getting warm weather crops in earlier, quite a bit earlier, I would use plastic mulch and low tunnels or something with greenhouse film or red tomato bags or wall o waters. In that situation, you do want to prep the soil some days before planting because the soil temp is the important thing.
I do use row cover to push planting up a couple weeks so if a cold snap comes it is ok. But my weather and soil are conducive to that method. Someone with a cooler climate would actually need to the warmth, not just frost protection like I do.

When it comes to fall, 2 layers of regular row cover will keep cold hardy veggies going into December for me. It is amazing.
I always use some 9 gauge wire hoops to hold up the row cover. Sometimes just a few and sometimes a lot. You buy the wire where the chain link fence is sold and a roll makes many hoops.
I use the new and holeless row cover on things that get bugs like cucurbit family crops, brassica crops, potatoes, peppers maybe. Then the older stuff with holes I have around I use for animal and frost protection on sweet potatoes, beans, peas, tomatoes, beets etc.
Yes you have to take it off when you get into summer. I find aphids go crazy on the heat bothered plants. I have had aphid problems on summer covered peppers, eggplant, rutabagas, napa cabbage etc. Even with a lightweight thin layer.
I use stones or boards etc to weigh the edges. Burying is not recommended but I sometimes do that for squash and melons that will be left for weeks.

One other thing, I bought some from M Leonard last year and the stuff disintegrated after a few weeks. It was horrible stuff. I cannot exaggerate it!

Here is a link that might be useful: farmtek row covers.

I am talking about row covers - but since they add a "small" greenhouse effect, I wondered if they could ALSO serve to warm soil, or, keep it warm.

Actually, what I was thinking is this:

Lie clear plastic down on the soil about a month before planting time to warm it up, then after planting, switch out for floating row cover, to try to maintain the warmth and give the plants a few degrees boost in temperature.

Would floating row cover allow me to start corn a few weeks early?

If I were to do that, should I use a "hoop" setup to hold the covers a bit higher?

Also - can I just drape a large row cover right over the hoops rather than attach the cover directly to them?

I would use black plastic not clear, for the weeds. Then add some mulch on top later.
With tomatoes you have to be really careful not to break them since they are so brittle.
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This shows how I handled my tomatoes and peppers/eggplant. The tomatoes I cage right away but if you put row cover over that it will rip. So I could use plastic film, wall o waters or these red tomato bags if the spring isn't too cold. I used red tomato bags from Territorial on the early planted tomatoes and then planted the rest mid May. (I am doing a low tunnel this year for them and one for peppers).
The peppers had some good hoops with row cover. They don't break as easily.
Starting tomatoes early is a big dilemma due to the cages. If you don't cage right away it is difficult later. If you do cage and stake the cages down, it will make covering them impossible. I have tried various things for many years!

Wind is a problem so I use paper binders on the hoops to hold the row cover on and lots of heavy stuff on the edges.

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Corn planted quite early.
I also started that corn inside. My goal was to have the first corn at market.

Clear plastic warms the soil much more than black. Black plastic has to be in direct contact with the soil, and radiates heat to the ground. Clear plastic acts as a greenhouse and traps heat from the soil directly. i tried clear plastic last spring to get my tomatoes and peppers into the ground and it definitely worked. In fact, you have to be very careful because it works too well. I kept taking the temperature of the soil and on sunny days had to lift the plastic to avoid baking the soil. At the same time, I was covering the plants to protect when frost threatened. Eventually, I tore up the clear plastic and put down black plastic down, cutting and overlapping it around the plants. Worked out very well, but it requires close monitoring, and i was home all day.