What are some recommendations for planting garage rooftop window boxes?

Question

The situation: There’s a double garage (precast concrete) with a flat roof of roughly 30 m² (approx. 320 sq ft), and it’s unused space. I often thought about putting anything alive up there (preferably of the kingdom Plantae), the one downside being the necessity of a ladder to look after whatever’s up there.

Regarding weather: While the garage finds itself located…

• in temperate climate (hardiness zone 8a), …
• it’s frequently overcast and drizzly, with 16 rainy days per month all year and…
• 4-6 sunshine hours daily between May and September with…
• average temperatures ranging from 8-18°C (46-64°F) in May and 13-24°C (55-75°F) in July.

Now I look for specific recommendations to plant greenery atop the garage. Suitable candidates will have to adhere to some considerations:

• I don’t want to put tons of soil or raised beds on it (excess floor load, max. 200 kg/m² ≈ 370 lbs/sq yd), so I figured window boxes (with drainage) would do nicely. They’re easy to provide, align and (re)move.
• Plants will have to endure full sun, storm, heavy rain or even hail. While these conditions do apply to the garden below, exposure is literally elevated and the surface might get a lot hotter (tar paper). Building a canopy for shelter is too expensive yet manageable if required.
• Since there are no stairs, plants shouldn’t need to be tended to permanently. Some climbing is okay though, like checking every few days or so.
• Are there any robust crop plants that thrive in window boxes (i.e. limited width and depth)? Alternatively, what ornamental plants make sense on a garage roof?

To be clear: I do not intend to permanently vegetate the entire roof area. It should be limited to a few planters during the growing season. I think it’s a bit different from a “typical” rooftop garden, which is a regular garden on top of a building.

Answer 1

Unfortunately, there are two things that might scupper your plan - the weight bearing capacity of the roof, and the necessity to water daily or twice daily when the weather is hot and dry, or simply just hot - very little rain gets into containers when the plants are in full leaf in summer. Whether you intend to actually walk on the roof or not, planted up containers with damp soil are surprisingly heavy, and it may be that the roof is not appropriately built to take any kind of weight.

UPDATE:

Let me just reiterate that I am not recommending placing containers on the roof you describe, you do so at your own risk. However, you've asked for plant recommendations for window box or trough type planters. Because the depth on those isn't usually more than 6-8 inches, and that means you're restricted to small plants rather than good sized shrubs, the best thing from an aesthetic point of view is to plant them twice a year - once in late spring/early summer with whatever summer annuals are available where you are, in particular those that trail (surfinia petunias, trailing lobelia, helichrysum petiolatum for instance, combined with upright pelargoniums - don't use Fuchsia, they won't tolerate insufficient water, but pelargoniums are pretty good about dryish conditions), and then disposing of all that in Fall and replanting with a winter display, again something that will only need to last six months. Not sure what you can get in your area for winter, but here, garden centres sell small versions of evergreen shrubs such as Euonymus aureus, Osmanthus heterophyllus 'goshiki', Skimmia japonica, dwarf conifers, so two or three of those in a box with something like pansies and some small trailing ivies, maybe some bulbs such as Narcissi 'Tete a Tete' or dwarf tulips. You will need to replace the potting compost in the troughs if anything becomes diseased, particularly fungal disease such as Pansy Downy Mildew, and the compost should be changed bi yearly anyway, though if there hasn't been disease or infestation by soil inhabiting insects, then you can leave it a bit longer. Note that the summer bedding will look much better and get much larger if you feed it regularly, on a weekly basis, with a liquid or soluble fertiliser (something like Miracle Gro all purpose is fine), and if they are given sufficient water for their needs.

You might be able to grow some of the smaller tomatoes instead in summer, but there is some difficulty with this - because of where they are, you are likely not to be able to give them the significant amount of care and attention they require in terms of water supply as well as monitoring for pests,diseases and nutrient deficiencies and treating/controlling any that occur.