That depends on what you are using them for. Row covers go on top of the plants to keep off insects and protect the plants from cold or in some cases to provide shade.

Black plastic and mulch go on the ground and are used to either pre-warm the soil in the spring or to keep it cool in the summer heat, to help retain soil moisture, and to keep down weeds.

They all have different uses and which may be the most effective and when they should be applied or removed also depends in part on what your climate is. So please, once again, include your garden zone/location in your posts. It is easy to do and is important information for us to have when answering questions.

Dave

I'm growing tomatoes, broccoli,cucumber and bell peppers I live in cornish Maine zone 5A I believe

Maine zone 5A

Ok but you need to put that in the box provided for that info when you post or fill out your profile page so that info comes up automatically. That way we don't have to ask for it.

But you still didn't answer the question about are you looking for row cover use or for mulch? They are different things. So I'm going to assume you want to mulch those crops rather than use row covers on them.

You can put down black plastic landscape fabric as a mulch. It comes with perforated holes in it to let water and air in and let heat escape. But using it can make watering the plants more difficult and end of season you have to rip it all out.

Or you can mulch around the plants with straw or hay or dried grass clippings or compost or even shredded paper. Lots of natural mulch options are available and they don't interfere with watering and end of season they can just be turned into the soil and improve the soil.

In your location if you use the black landscape fabric it will probably not over-heat your soil as it can in the warmer zones of 7-8-9 but you will to keep an eye on the soil temps in mid-summer. If it does get too hot then you need to cover it with one of the natural mulches.

There are lots of discussions here about all the types of mulches that the search will pull up for you or you can go over to the Soil & Mulch forum for even more info.

Dave

Bell peppers could probably benefit from plastic mulch.

Tomatoes and cucumbers like slightly cooler soil temps. and would work with organic mulch like straw.

Broccoli - definitely mulch with organic mulch to keep soil cool and use insect barrier grade row covers to keep cabbage worms and aphids off.

The effect of plastics(similar to trash bags) is not so much for absorption of solar heat but slowing down the soil underneath from cooling due to convection(air movement). In effect, it keeps more of ground's own heat coming up. Average ground temperature is about 65F. which is almost perfect for most plants. So in the winter it cools down(even freezes) down to several feet and in the summer(depending on location) warms up to several feet.

I think, therefore, technically, plastics, black or any color, when tightly sitting on the ground, also traps the moisture. So as the surface temperature rises, the warm moisture penetrate deeper into the ground. That is the mechanism of heat transfer due to plastics.

I am not for plastics, because they suffocate the ground. I think the soil should get both sunshine and air. That is why the old tradition of hoeing makes sense. It might be hard work but it has benefits.

In other words, plastic row covering for the purpose of weed prevention and moisture maintenance, in hot summer days just not in harmony with the nature of the plants and the earth underneath. JMO, that's all.

Let me share my experience with row covers and plastic mulch. BTW, these are both new to me this year.

I have 4 raised bed, each 4'x 10'. For my cold crops, which I planted in early march (well before the last frost) I used hoop covers during times when the temperature dipped into the mid-twenties or less. I took it off during the day. I haven't used a row cover for them for about 5-6 weeks now and they have done great.

In another bed I have a combination of cold and warm weather crops. I planted mid-late April and even though I used a row cover, one cold windy nite zapped all of my peppers and tomatoes in this raised bed. I replanted and now all plants are doing well, including cabbage which is a new vegetable for me.

My other 2 raised beds are strictly warm weather crops (tomatoes, squash, melons, etc). They have IRT plastic mulch and row covers. These 2 strategies kept the crops in a warm environment even with cool daytime highs and they are doing great. I plan on covering the plastic mulch with organic mulch sometime in the next 2 weeks... depending on the weather.

Tom