Rooftop Harvest

The first apartment Michael and I shared was a fifth-floor studio walk-up on the Upper West Side of New York City. We told ourselves it was a test: If we could survive living in one room we could get through just about anything. (Spoiler alert, we just celebrated our 25th wedding anniversary, so it worked out pretty well.)

In warm weather, our living space doubled because where the apartments below had a bedroom, we had a terrace: a generous piece of open space where we spent most of our time from April to October. You may not know this, but if you have a terrace in New York City you are contractually obligated to grow tomatoes on it. OK, not really, but why wouldn't you? Tomatoes love the heat and full sun of rooftops and terraces. Unfortunately, we were both starving artists with severe budgetary constraints so we had to get creative.

Our solution: we planted grow bags. Grow bags are perfect for decks and terraces where you can't use permanent boxes (too expensive, too heavy, too large). Years later we used them on the ground the first summer in our new house because we hadn't decided where to put the vegetable garden and didn't want to rush the decision.

The fastest and easiest way to get started is to buy large bags of potting mix (2-3 cubic feet) and plant directly inside them. If you're growing on a deck, use a lightweight, professional mix or a potting mix that's part soilless, peat-based mix and part potting soil. If you're growing on the ground, you can use top soil.

The number of plants you can grow in each bag depends on what you're growing. Since tomatoes are heavy feeders, use one bag per plant. If you're growing cherry tomatoes, you can probably squeeze two plants into a bag with 2-3 cubic feet of soil. Basically, give each plant the same amount of space that you would in a traditional garden plot.

Lay the bags flat on the ground; then use a sharp knife or box cutter to slice an X in the top of the bag for each plant, and fold the flaps back. The X's should be just large enough to accommodate the root balls of your young plants.

Since the potting mix in a grow bag is lightly packed, planting is pretty darn easy. Just use your hands to move aside enough mix to make room for the root ball. Insert the plant, then firm the soil around the roots. Use the cutting tool to punch a few holes along the lower edges of the grow bag -- one every six inches -- and a few more on the bottom of the bag. This will allow excess water to run off. Next, water your plant, thoroughly soaking the entire bag of soil.

Grow bags make efficient use of water, since the plastic is nonporous (aside from the holes you just poked in it) and retains moisture well. A lightweight peat-based mix dries out more quickly than a bag of topsoil. You'll have to feel the mix regularly to get an idea of how often you need to water. And remember, rain can't penetrate the plastic so you'll have to do all your own watering.

How often you feed the plants in your grow bags depends on what kind of soil you use. Top soil contains more nutrients than lightweight soilless mix and needs less frequent feeding. Plus, since you'll be watering top soil less frequently (remember, it stays wet longer), soil nutrients will be leached away more slowly. A good general rule is to feed plants in a soilless, peat-based mix once every two weeks and plants in topsoil once a month.

Tomato cages and stakes can be stuck directly into the grow bags. If your bags are on the ground (as opposed to a rooftop or deck), give your plants extra support by pushing the cage or stake all the way through the bag into the soil below.

And here's another great thing about grow bags: Since you plant new ones each year, you don't have to worry about exhausting the nutrients in your soil or overwintering spores from fungal diseases. Just take them to a local yard waste recycling center to empty their contents or open up the grow bags at the end of the growing season and add their contents to a garden bed.

Tomatoes are the perfect plant for rooftop growing. They thrive in the heat and wind, and with a large enough grow bag (or any container, really), they'll never even notice they're not in solid ground.

Ellen Zachos is the owner of Acme Plant Stuff (, a garden design, installation, and maintenance company in NYC specializing in rooftop gardens and indoor plants. She is the author of numerous magazine articles and six books and also blogs at Ellen is a Harvard graduate and an instructor at the New York Botanical Garden; she lectures at garden shows and events across the country.

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