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How can I maximize solar radiation for sun lovers?

For example, in central Germany, you'll have some summers that won't have a lot of sun and/or heat, yet you want big peppers and good grapes.

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    I will later or tomorrow post some strategies myself. Typing a question like this into websearch does not yield good results for me, but I think this is pretty useful.
    – phresnel
    Jul 30, 2014 at 20:29
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    Greenhouses or polytunnels.
    – Bamboo
    Aug 2, 2014 at 15:12

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I live in a spot with short but hot summers, so it is possible the solutions that work here will be less effective where you live, but here is what I'd suggest:

  • If you have space to do so, you could try building a solar trap. This is a u-shaped garden that opens to the south. At the north end of the 'U' are the tallest plants. Form the sides of the U with plants that get progressively shorter. Plant your sun-loving veggies along the north side of the U, but south of the tall plants that form the bottom of the U. Does that make sense?
    • If you have a brick or stone wall that faces south, you could plant the heat-lovers right up against the wall to take advantage of the thermal mass. The wall will absorb heat and radiate it back at your plants. If you do not have this, but have access to boulders or bricks, you could build a little stone wall or alcove behind your plants to do the same. Or if you have a light-colored wood fence or wall that faces south you could do the same, as the light will reflect off the wall and back at the plants.
    • You could mulch with plastic sheeting. From what I understand, red plastic works the best, but any plastic will raise the temperature of the soil around the plants and help your heat-lovers out.
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If your goal is season extension (start earlier in the spring, extend it later in the fall) then solutions such as a hoop house, high tunnel hoop or cold frames are helpful. Here in Virginia (zone 7a) I have about 6 months of frost-free growing but the hoop house helps by keeping the cold, dry wind off of the young plants in the early spring.

A south-facing brick or concrete wall will heat up from the sun and slowly release that heat later. That may help grapes grow better if they were planted on a trellis next to the brick wall. Similarly, it would create a "micro climate" for other plants (e.g., those peppers) that specifically like the warmer conditions.

While I haven't done "hot house" tomatoes (yet), the local PYO (pick your own) farm we frequent does this and the tomatoes are much farther along than mine which are started under the lights in the garage and moved outside later. Their greenhouses create a very warm environment that causes those tomatoes (and a few other plants) to thrive.

In my hoop house, I position a mylar sheet along the inside of the hoops on the north side to reflect light back at my plants within. The sun never shines inward from the north side (since I'm at 37 degrees North) and so I don't block any direct sunlight by doing this. Instead, it uses that light which would otherwise pass through the north side plastic. That's the theory anyway.

Here in VA we have (generally) sunny, warm summers that are great for growing peppers and other heat-loving plants. In an environment where things are cooler, containing some of that solar energy helps to effectively push the "hardiness zone" southward - I think Europe has hardiness zones like we do in the States. So trapping some of that heat in a plastic tunnel can help to improve conditions for heat-loving plants.

Of course, when you are using a plastic tunnel you've got to be aware of overheating and conditions being too dry (due to rain water being blocked), but that is manageable.

One other thing - in my hoop house I have black weed fabric on the floor which helps to warm the soil under it. That definitely helps.

Just some numbers - and while you're not specifically asking about winter planting in a greenhouse or hoop house, I think it might be useful info: on average my winter nighttime temperatures were 5-12 degrees F warmer than the outside ambient temperature. My soil temperature inside the hoop house never got below the mid-40's despite the fact that we had lots of near-zero F nights. Daytime temperatures would be 15 degrees warmer on overcast days on average and 35-40 degrees warmer during the day. On one sunny day when it didn't make it out of the 20's degrees outside, it was a balmy 68 degrees inside. Certain plants loved that.

Anyhow, enough rambling. Containment of the warmer air and thermal mass such as bricks and stones help create warmer conditions. Even in the summer, hoops can be used to heat things up. It just requires close monitoring of the temperature and moisture levels.

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