I cover chrysanthemums with black plastic in 100F+ degree weather to induce flowering, I'm sure it work just fine with cannabis.The tarp guy has failed to consider certain unavoidable facts. If you place plants under a dark tarp, in direct sunlight, you have just created a solar oven which will crispy your plants like no bodies business. If you're wanting to avoid detection, then I agree with the tie them down guy. Be careful not to break your plants, but tying them down has the added benefit that your plant will fill the tied portion with resin and other goodies in an attempt to pull itself back up.