warming the soil - black covering?

[ChingfordHarry]

Hi

I'm planning on growing melons this year in a 4 x 1m plastic covered tunnel/cloche (last year they were okay without, but if I can improve things that would be, well, an improvement).

I've just heard on GQT that it helps if you cover the soil with black plastic to warm the soil - but I'm left wondering about which black plastic - impermeable polythene sheet or black weed-suppression fabric (which might allow the soil to breathe)? Either would need holes cut for the plants and presumably to water/feed; they seem to go for broadly similar prices. It occurred to me that I could use a roll of black binbags, but these would be somewhat flimsier.

The RHS website says that clear plastic is better for pre-warming soil, but a cloche on its own is better for clay soil (which is what I've got - for fun I try to find the soil in the clay...) until it dries out.

Are there other options? What are people's personal preferences? Why?

[ChingfordHarry]

whoops. Just noticed - the statement above that starts with "last year they were okay without..." is in the wrong paragraph, and actually refers to being grown without black plastic covering the soil - the melons were grown in the polythene-covered cloche/tunnel, and I used cardboard to cover the soil to try to keep weeds down.

[Jungle Jane]

Heat goes through clear plastic better,allowing shorter wavelength solar radiation to enter into soil (I read that scientific bit online ) I’ve got a clear bag over a pot of peas,I’ve also got net curtain over another,same variety,same amount of peas,the clear plastic is locking in the heat more than the net but I wonder,they’ll probably all come up at the same time grass cuttings are a good mulch to stop weeds,plastic hides the slugs…

[ChingfordHarry]

Yes, a clear covering will let more visible light through; if you go a step further and add a black layer close to the ground (if you're not lucky enough to have black soil) it will absorb more of that light than other colours would, and this absorbed light energy is re-emitted as heat (basically infra-red light, which has a longer wavelength than visible). Of course, other colour surfaces will also re-emit the light energy that they have absorbed as IR, but some of it isn't absorbed at all, and is just reflected as visible light; I wouldn't expect the warming effect to be as great.

However, I think I've managed to confuse things - I was really after a comparison of impermeable plastic and weed-suppression type fabrics as the black layer, or for advice on other options. Sorry to have caused any confusion!

[ameno]

I grow melons outdoors every year, planted through black plastic as you are planning, no cloche or tunnel. I grow Emir.
I consistently get a pretty good crop, usually 12-14 melons, each weighing 750-900g, from a 6ft x 4ft space.
You could likely save space either by growing them up something (although they need more water, not to mention support for the fruit, if grown vertically) or by allowing them to weave in between other plants, although obviously you wouldn't be able to cover them that way.

Definitely use black, not any other colour. Even lighter colours, much less clear, let in too much light and you'll have weeds aplenty germinating under the plastic.

As for which type to use, I recommend the impermeable type. It will require a bit more watering, as only so much rain will get into the holes, but it lasts much longer (as long as it's designed for outdoor use and thus UV stable). The woven plastic stuff starts fraying after just a year or two, and the ones which are basically a thick black fabric are even worse, as they disintegrate in no time.

You don't necessarily need to buy any, though. If you only want to cover a small area, you can cut open old compost bags (as long as they are black on the inside) and use those. Don't use bin bags, though. Not tough enough, and not UV resistant enough.

Also, be sure to bury the perimeter of the plastic under the soil (just a couple inches around the outside is enough), rather than pegging in. It resists the wind better (as the wind can't get under the plastic to lift it), and more importantly, it keeps the slugs out. If you just pin it, thus leaving gaps around the edges, it will soon become a haven for slugs.

[ChingfordHarry]

Hi Ameno

many thanks for this.

The only issue is that I need all my old compost bags for collecting broken glass, old ironwork and other sundry "archaeology" from the previous plotholder's shed fire that I am still excavating after several years on site, so I think I will need to buy the covering!

[Jungle Jane]

I grow melons outdoors every year, planted through black plastic as you are planning, no cloche or tunnel. I grow Emir.
I consistently get a pretty good crop, usually 12-14 melons, each weighing 750-900g, from a 6ft x 4ft space.
You could likely save space either by growing them up something (although they need more water, not to mention support for the fruit, if grown vertically) or by allowing them to weave in between other plants, although obviously you wouldn't be able to cover them that way.

Definitely use black, not any other colour. Even lighter colours, much less clear, let in too much light and you'll have weeds aplenty germinating under the plastic.

As for which type to use, I recommend the impermeable type. It will require a bit more watering, as only so much rain will get into the holes, but it lasts much longer (as long as it's designed for outdoor use and thus UV stable). The woven plastic stuff starts fraying after just a year or two, and the ones which are basically a thick black fabric are even worse, as they disintegrate in no time.

You don't necessarily need to buy any, though. If you only want to cover a small area, you can cut open old compost bags (as long as they are black on the inside) and use those. Don't use bin bags, though. Not tough enough, and not UV resistant enough.

Also, be sure to bury the perimeter of the plastic under the soil (just a couple inches around the outside is enough), rather than pegging in. It resists the wind better (as the wind can't get under the plastic to lift it), and more importantly, it keeps the slugs out. If you just pin it, thus leaving gaps around the edges, it will soon become a haven for slugs.

Slugs can get into the holes like rain can,I don’t use any slug pellets in my garden,I use straw as a mulch around melons allowing the blackbirds etc in to find pests & stops weeds. Slugs love plastic to sleep against. It is just personal preference.

[ameno]

Slugs can get into the holes like rain can,I don’t use any slug pellets in my garden,I use straw as a mulch around melons allowing the blackbirds etc in to find pests & stops weeds. Slugs love plastic to sleep against. It is just personal preference.

They can, yes. But they are less likely to and will get there more slowly, especially early in the season when there is very little plant cover, as they don't like moving across wide open spaces when they can avoid it. And of course early in the season, when the plants are just transplanted, is when your plants need the most slug protection.
There is no reason not to do all you can to deter them, especially as burying the edges is pretty easy, saves on the number of pegs you need, and makes it less likely to blow away in the wind.

I grow extensively through black plastic, carefully tucking under the edges, and by the end of the season I only have a relatively small number of slugs under the plastic.
A couple plot neighbours tried growing through black plastic, but pegged the edge instead. Their plastic had dozens of slugs under it by the end of June, and everything they planted had been eaten.