Plastic needs to be in direct contact with the soil to warm it. With hoops all you are doing is warming the air, containing the heat that the soil gives off.

Whether black or clear works best is debatable.

Dave

I don't think clear plastic needs to be in contact with the soil. It doesn't absorb sunlight. It just holds in the warm air from the sunlight-warmed soil. Black plastic certainly does need to be in contact with the soil because IT absorbs the sunlight, not the soil underneath it. The soil underneath black plastic never sees sunlight. The black plastic gets warm. The clear plastic does not.

Good question about air volume under clear plastic. Air is a (poor) insulator. In principle, I think the more air the better. Increased air volume helps. The clear plastic itself is cold. So you'd like to put some insulation in between it and the soil. But you have to have a good seal to keep the warm air in and the cold air out. Of course, a good tunnel should have that seal.

You read correctly, Isgen. Everyone else is blowing smoke.

Makes some sense that clear plastic is better for warming soil than black plastic, simply because it's really hard to be sure the black plastic is in good contact with the soil everywhere. Soil is already pretty dark, so I suspect that black plastic won't absorb a lot more sunlight than the soil itself would.

Could row cover/Agribon be used in place of the clear plastic and the same effect be achieved? Though I understand that row cover/Agribon will not warm to the same extent.

Dan - Agribon is a frost blanket designed to keep warm air in/cold air out. Depending on the weight it can provide between 3-10 degrees of protection. But I don't think it would do much good for actually warming the soil.

I use clear plastic, fondly hoping it will produce a greenhouse effect.

My apologies. I was thinking Agribon was for pests. But if it is transparent, and holds warm air in, sure, it ought to work fine. I deleted my comment, since it was incorrect. Is Agribon, in fact, transparent? It looks white to me, meaning it is reflecting a lot of sunlight.

Some Agribon is exclusively for pests - the insect barrier grade. It comes in many different weights but none of them are clear. They are all shades of white depending on the thickness and all weights are air and water permeable so no, you couldn't "use in place of the clear plastic and get the same effect " as using plastic.

Can you get some soil warming from tunnels like isgen said, sure. No one is disputing that. But the original question was

"would I get better results pinning the plastic right to the ground, or putting up hoops and making a low tunnel "

and it's well proven that better results - faster and higher temps - come from direct contact.

As to clear or black, as I said it is debatable. If for no other reason than black, constructed exclusively for this purpose, is more readily available and thousands of miles of black, not clear, are used by the AG industry annually.

Does that in any way mean you can't use clear? Of course not. Not unless your secondary goal is weed control.

So if that is "blowing smoke" so be it.

Dave

Agribon is product with differing levels of transparency, permeability, and insulating factors. Their lightest is simply an insect barrier. It is very permeable, and provides very little insulation. It's like 90% transparent.

Agribon, of any thickness, is not what you're going to want if the goal is to warm soil. If anything, it will conserve the present heat of soil, but will not, in any way, work to warm soil. In fact, it will do exactly the opposite. The thicker the agribon, the less of the sun's heat will reach the soil.

I'm in a similar climate where the weather patterns are changing. There is no traditional spring here anymore. One strategy I would suggest, and I've been employing it for a long time, is to cover your beds with lots, and lots of leaves. This will act as a blanket. However, when you want the soil to warm, you have to remove the blanket - and be ready to rake it back over if an early spring freeze comes your way.

With this technique comes fall preparation... and this is what I'm going to have to switch to for all my early spring crops. No more spring digging for early crops. Prep the soil in the fall. Cover very thickly with the mulch (using the summer cages to keep the leaves in place.) Then, no digging when spring comes. Simply plant. If the ground is frozen, or too cold... clear plastic for tunnels, or clear or black for temporary warming.

Without this method, I'll pretty much have to consider moving all spring crops to fall only. In fact, our climate is warming to the point of following the methods of the deeper south... over-wintering the crops that are traditionally spring veggies in the north.

ETA... Dave was typing at the same time...