Answer: Rooftop gardens are quite difficult in that they are windy, hot,sunny and consequently also offer dry conditions for plants. In New York, there is the added problem that winters are relatively cold for plants growing in containers rather than planted in the ground. For this reason, it will be difficult to get plants to survive the winter.
Evergreens are especially susceptible to wind burn and drying out during the winter months, so you could experiment with these but please understand they are risky. I would suggest you try junipers and see how they do for you. (Although you may see evergreens here and there in containers, in many cases the container evergreens in the city are routinely replaced in March.)
You might also try a smaller growing variety of a deciduous tree such as a crabapple if you want to add height and winter interest to the rooftop garden. These tend to do relatively well in large containers.
Larger containers are always better because the increased soil volume holds moisture better and allows more root space for the plants. This helps keep the plants hydrated during hot or windy weather. I would suggest no smaller than a half whiskey barrel size for this reason.
Also be sure there are working drainage holes in the bottom of the containers to allow excess water to escape. Be sure you have an easy way to water, as during the heat of summer you will need to water daily if not twice a day.
Use a good quality potting mix formulated for container plants. Plan on fertilizing regularly from early spring through early fall with a slow release fertilizer such as 10-10-10 plus minors and/or a water soluble fertilizer with a similar analysis. Topdressing with compost and using an organic mulch on the soil surface will also be helpful.
If possible, provide a windbreak.
I would suggest you try some annual vines such as morning glories (bloom in the morning) and moonflower vines (huge fragrant white flowers open at night) planted from seed each spring. Or, you could try trumpet vine (Campsis radicans) which is a strong grower with terrific summer blooms, however the drawback is that this vine leafs out rather late in the spring. These will all easily climb the wire fence. If you want to hide the ugly fence year round, you could use lattice. This is lightweight and provides a good climbing support for the vines to twine through.
Wet potting soil is very heavy, so I strongly suggest you check the weight capacity of the decking and roof before launching into this project.
You may also want to take a look at what other rooftop gardeners with similar locations are succeeding with and learn from them. I hope this helps with your planning!
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