Grapevine on a Deck We've recently moved into a house with a lovely south-facing 10' x 20' (exposed-aggregate) concrete deck. This deck is on the second level of a 3-storey house. It is soundly supported, and drains into the existing gutter system. We would like to make plans to build a pergola to cover most of this deck for shade, and to serve as the trelliswork for a vine of grapes. I would like to know the following: 1. Would CEDAR be the proper choice of building material for this pergola? Or would the more economical TREATED WOOD suffice? Would the TREATED WOOD leach harmful chemicals to the grape plant, and thus render the fruits not suitable for human consumption? 2. How much of a root system this size of a grapevine will need, so that I can build a properly sized and dimensioned (I'm concerned about the depth & breadth thereof), insulated, PLANTER BOX? 3. What type, or composition, of soil mixture is appropriate for this purpose?
The vines can be quite happy in containers, in fact most of the vines in the CFIA repository in Sydney are in (large) pots. Grapes "au naturel" have extensive root systems so there are some compromises to be made in your expectations as they will have a limited ability to absorb nutrients compared to field grown vines. Something around 20 gallons (actual capacity) would be a minimum. That's about 1/2 of a 45 gal. drum and in fact the blue plastic drums can be cut in half for such use. Drainage is vital, and at the same time, they will consume a huge amount of water when fully leafed out. Ralph
I'd go with cedar, without question for the container, and preferred for the pergola. The leach radius for treated posts is said to be 6", but in a container that's way too much. The pergola lattice will be in close contact with the growing vine, where trellis posts are not (the vines are trained along the wires, not the post). You will also find that treated wood quality is generally low, much lower than any reasonable grade of cedar, and the PT wood will rapidly check and visually deteriorate even while it may "last" for some time. I'd start with a soil volume of 2' x 2' x 2', with the structure and insulation (do you really think you need insulation? I'd think not) outside that. A premium potting mix.
OK, CEDAR it'll be. (Actually, I'd just needed some encouragment to go ahead with the idea). Now, should I consider using (Thompson) WaterSeal on the cedar, or should the beautiful wood be left bare, and untreated? Would the WaterSeal-on-Cedar also have the same harmful leaching effect on the grape plant, as with Treated Wood?
My parents have been growing grapes on a second-floor deck for years, but they planted the grapevines in the ground and trained them up to the deck. This strikes me as the best option if you can do it. I stay away from that treated wood too; it isn't just the leaching, but the sawdust when you cut it that poses unknown effects. Maybe you're allergic to cedar too, though, so maybe a dust mask is a good investment either way :-) I don't know about the safety of Thompson's, except that I've never heard of any risk. If you don't seal it, the cedar will weather to grey.
The deck is surround by concrete sidewalk on all 3 sides; thus, planting in the ground option is out. I was speaking to a contractor today about the Thompson WaterSeal. His comments were that I should, instead, look into the type of WaterSeal that is actually "water" based, for the reason that anything that is solvent-based might give off chemicals in the heat. Another option he suggested was to used a short 1" U-shape aluminum cap at the top ridge of the cedar, to prevent water penetration; and then to leave the cedar bare, and untreated. I was also in conversation with a friend over the weekend, who also happens to be a contractor, about the construction of this pergola. His suggestions were: (1) use 2 x 8 or 2 x 10 cedar boards, standing on edge, at 20" or 24" on center, with 2 x 2 bracing at the midway point (to prevent any twisting). The higher profile of 8" (or better still 10") will provide for a more effective shading in the afternoons. (2) Furthermore, instead of using 1 x 2 cedar to make up the rest of the trellis work on top of the 2 x 8 (or 2 x 10), use stainless steel 12-gauge wire, stainless steel eye-hooks, and stainless steel turnbuckle at the ends, to loop a network of trellis. (3) In view of the extra weight of the 2 x 8's (or 2 x 10's), and also of the weight of an abundant crop of grapes!!! please make sure the support post & beams framework above the existing railing of the deck is sound for the job.
