Research Guides: Urban Gardening: Home

Rooftop Garden in New York City; photo courtesy of Flickr cc/Queena

City gardens present exciting challenges. Faced with pollution, variable microclimates, heavily compacted soil and limited space, the city dweller needs to be a resourceful gardener. Despite all this adversity, beautiful gardens can be created. Below are some tips and considerations for the urban gardener.

Design Considerations

Before creating an urban garden, decide how you are going to use the space. Is it an extension of your home that needs to look nice throughout the year, or is it a seasonal garden that can peak and fade?

Space Savers

Cluster plants together, using different sized pots to create an attractive display. Maximize your use of space by under-planting trees and shrubs with annuals and perennials. Decorate walls with trellises. Grow ornamental vines and climbing roses on trellises, or simply use them as decorative elements to hide an ugly wall. Create different gardening rooms to give the illusion of space. Turn a storage or tool box into a bench by placing cushions on top.

Wind and Privacy Screens

Trees can be used as wind and privacy screens. Stagger plants so they have room to grow. Leaving space between trees is particularly important on rooftop gardens to ensure that they do not act like sails, taking the full force of the wind. Build partition walls or fences to cut back on wind and create privacy. For wooden structures, redwood, cedar, teak and cypress are all rot-resistant woods that do not require preservatives. Cheaper woods such as pine will need to be stained or painted to last. Shade cloths, bamboo screens, trellises and awnings all serve as sun screens.

Space Restrictions

Remember to measure doorways and any spaces that need to be navigated in order to reach the garden. Measure the dimensions of elevators and stairways (height, width and diagonal) before buying material to be transported to your rooftop garden.

Building Codes and Weight Restrictions

Make sure to investigate approved surface materials for your rooftop garden; decking and tiles (plastic or quarry) are two possibilities. Pay attention to weight (typically 30 to 40 lbs. per sq. ft.) and height restrictions. Dunnage and decking are two ways of spreading the weight of containers. Wooden decking is often restricted by fire codes (generally allowed to cover 40% of the area). Decks may need to be cushioned by fiberglass or asphalt footing so that they do not dig into the surface of the roof. Pay attention to drainage issues and make allowances for access to repairs.

Irrigation

Watering is important in a rooftop garden. Containers need to be watered daily when the weather is hot and water pressure needs to be good. A snap coupler can be purchased at the hardware store and attached to a sink in order to allow you to run a hose from your faucet. A better alternative is to invest in drip irrigation that comes with a timer, water pressure regulators and a whole host of gadgets.

Reflected Light

Reflected light from light-colored walls or large areas of glass can add to the intensity of the environment. Try painting the wall a dark color (gray, blue, deep red and green absorb light). Covering a wall with vines is another effective way of cutting back on the glare.

Plant Selection

City gardeners need to find tough plants that grow well in confined spaces. Foliage and texture play an important role in urban designs, where low light levels and limited space often dictate plant choices. The best way to discover what will grow successfully in your garden is to visit local parks and explore your neighborhood to see what thrives. For a list of plants that are suitable for many city planting sites, refer to the "Plants for City Gardens" tab at the top of the page.

Planting Containers:

Choose a suitable container for your location. Plastic or poly resin containers are lightweight and easy to move. Terracotta breathes well and is aesthetically pleasing. It is also heavy, fragile and, unless it is thick and fired to a high degree, is not always frost resistant. Wooden containers will need stains and weatherproofing depending on the wood used.

The hardiness rating for New York City is zone 7, but rooftop gardeners are often advised to buy plants with hardiness ratings between zones 5 and 6. Drought- and wind-tolerant plants are good choices for these harsh conditions. Dwarf varieties that do not overwhelm the space or require heroic pruning measures are also well suited to these small urban spaces.

For over-wintering plants, the container should be at least 14 inches wide and deep for the New York area. Small trees should be planted in 24-inch containers, shrubs a minimum of 16 to 20 inches, and perennials 14 inches. There are always exceptions to the rule. Experiment and see what works.

Make sure that the container has several drainage holes. They should be ½ inch wide for large pots.

Add a drainage layer at the bottom of your pot consisting of Styrofoam chips, pebbles or pieces of terracotta pots (crock). Add one inch per foot of container height.

Add a barrier to prevent the soil from seeping through the drainage holes. Use a piece of an old screen.

Fill partially with potting soil mix. Potting mix can consist of store-bought potting mixes amended with compost or aged cow manure (4:1 or 3:1 ratio depending on how you like your soil). Alternatively use 1 part loam (topsoil), 1 part compost and 1 part potting soil (or perlite depending on your drainage needs).

Tease out the roots of your plant before planting. Position the plant at the same height as grown in the store bought container. Add more soil to the fill pot. Leave room for mulch and an inch to ½ inch of empty space at the rim of the container for watering.

Scratch in a slow-release fertilizer at the time of planting. Supplement with liquid fertilizer during the summer.

Mulch containers with shredded bark, cocoa hulls or any permeable mulch (pebbles, sea shells).

Place blocks or feet underneath containers so that they are not sitting directly on the roof or the deck; this makes for better air circulation and good drainage. Placing rubber matting underneath the container is another way of protecting the surface.

Maintenance Tips:

Amend soil in spring by either adding compost on top of the container as mulch or digging it into your container. Fertilize the container in spring, as above, and add more mulch if necessary.

Use trailing plants as edgers to keep roots cool in containers.

Divide perennials every 2 to 4 years to keep them to size. Use this as an opportunity to amend the soil.

Remember to prune trees and shrubs to size. Root prune every 2 to 4 years in early spring by slicing down the sides of the containers (just two opposite sides in any given year).