The mechanics of farming six stories up

When I tell people that I farm on a rooftop, there is often a pause followed by: “Wait, did you say a roof?” Next, most people ask how and why. Both good questions. When I started farming I never imagined that I would eventually be growing on a rooftop.

I started my business, Bluma Farm, in the fall of 2014 and grew my first cut flower crops in 2015. For the first four years I commuted an hour to my farm in Sunol, California, from my home in Berkeley. I knew early on that I wanted to move my farm closer to home. So, when I connected with Benjamin Fahrer of Top Leaf Farms who was looking for someone to take over his roof operation in Berkeley, I was ready to say yes.

rooftop-farming-the-riseJoanna Letz on her rooftop Bluma Farm in Berkeley, California. Photo by Emily Murphy


The rooftop farm makes a lot of sense in a city. Navigating land prices and leases on vacant lots can be tricky. Backyard farms can also be a viable option but finding enough backyards in proximity to one another to make it financially feasible is also challenging. Land in the Bay Area is a hot commodity and rooftop farms are a great way to expand  available city space to grow. In 2019, New York City passed historic green roof legislation. Hopefully more cities will follow suit. Currently, I am working with Fahrer to pass similar legislation in Berkeley. 

The building where I currently farm was completed in 2016. The farm was built at the same time the building was constructed. The developers, Dwight Neun Owner, LLC, were motivated to create a rooftop farm and worked with Fahrer to finance and install what was needed. I moved my farm and took over the roof operations in the spring of 2019. I have a lease that California FarmLink was instrumental in helping to create. Writing a lease for a rooftop farm on a residential building is more complicated than leasing typical agricultural land. 

rooftop-farming-the-riseMax Rosenblum constructing a sweet pea trellis. Image courtesy of the author.


On any given day, the Bluma crew and I are busy with tasks farms are familiar with: weeding, irrigating, planting, harvest and sales. I often am so immersed and focused on any number of these tasks that when I look up from work I am continually surprised to see clear out across the San Francisco Bay and down six stories. Sometimes mid-conversation with a crew member or visitor we are interrupted  by the sound of fire engines speeding down the street, horns blaring, or people rocking out to music on the street below. The bell tower in Berkeley rings every hour. When I’m not playing my own music, I look up at The Campanile and listen. 

rooftop-farming-the-riseA big bunch on the roof.


The roof is home to a whole host of small birds. The hummingbirds shoot past me while I’m working and I stop to watch as they feed on our salvias. The other day I looked up to see a hawk flying above which is an unusual site. Unlike other farms, we rarely see birds of prey because we don’t have small animals like gophers or mice. 

I certainly don’t miss gophers or mice, but I do miss seeing the larger birds like the blue herons, hawks, and eagles. Also absent on our farm are the snakes, frogs and lizards that I am used to seeing on the ground. We do have worms (some of which Fahrer’s son brought up to the roof from his home garden), insects (including pests like cucumber beetles), pollinators and microorganisms.


My farm business has thrived on the rooftop, outdoing my expectations. Growing on the roof is similar to container growing, but in my case, it’s like one large container. The building I farm on is actually 16 separate modular buildings connected by pathways. All together we have about ¼ acre of growing space including our perennial hedgerows. 

Each roof is formed like this: the bottom layer attached to the building is an impermeable waterproof layer of plastic (thermoplastic polyolefin TPO membrane), followed by a layer of polyester woven fabric, another layer of fabric protection, a layer of rock (lava rock and pumice), another layer of fabric, and finally the growing medium. The growing medium is a bit like a potting mix; it’s high in volcanic rock for drainage. 

rooftop-farming-the-riseChocolate cosmos on the roof. Images courtesy of the author except where noted otherwise.


As you can imagine the roof needs to drain easily and quickly so as not to be overburdened during a rainstorm or when irrigated. Fahrer also installed a French drain system. Essentially this is a perforated pipe that runs along two sides of the roof. When the soil is saturated the water runs into drainage vents and off the roof. 

The growing medium (soil) at its maximum is about 10 inches deep. It is a mix of of 5/16” lava rock, pumice, fir chip, compost, coco coir, bio-char and additional nutrients (see graphic for details). In some areas it can be as little as 5 inches. We add soil when we turn over our beds to replace soil taken off the roof during the growing season. We also add additional amendments such as feather meal and kelp meal each time we plant. Last year we added 3,000 pounds of additional soil that we brought up in the elevator in bags. When the farm was first installed the soil was blown up to the rooftop from the street with a large pipe and blower (and captured on YouTube videos if you’re interested). 

Managing soil in a container is very different from on the ground. We are not building soil health as I was used to. We rely heavily on our fertigation system (liquid fertilizer that runs through the irrigation). We run a light dose of fertigation most days of the week with California Organic Fertilizers Phytamin All Purpose 3.7-2.7-3.7, made from seabird guano, potassium sulfate, and fish solubles. Last year the soil mix we added to the roof was also high in biochar, and I noticed a lot of vigor in our crops. 

rooftop-farming-the-riseThe author harvesting. Photo by Nicola Parisi.


Instead of growing a winter cover crop we cover all roofs that don’t have a fall crop in silage tarps. When we lift the tarps in spring the beds don’t need much before we can plant. We re-shape, do a light forking, add some amendments and we’re ready to go. I also remind my crew when we are using spades and forks to use these tools at an angle to avoid ripping the fabric layer below the soil. 

I am often surprised how well the plants do in less than 10 inches of soil. During the summer we have an abundance of blooms. From July through October we harvest six days a week. Our biggest crops are: cosmos, chocolate cosmos, dahlias, zinnias and lisianthus, similar to the crop plan in Sunol. 

The weather in Berkeley is milder than in Sunol so I’ve adapted what I grow to best suit the climate. In many ways the cooler weather is better suited for flower growing. We grow somewhere around 40 to 50 different crops of flowers and herbs. This year I plan to grow smaller amounts of more crops and will try some new things, mostly edible flowers and some new herbs. Our edible bouquets and herb bouquets proved to be popular and I love growing them.

rooftop-farming-the-riseJoanna Letz with some of her chocolate cosmos.


Some crops do better on the roof than on the ground, notably our chocolate cosmos. I found on the ground it was harder to keep the small chocolate cosmos plants watered well and weeded. On the roof, it’s easier to manage this small tuberous plant and they also like the friable, aerated soil. In some ways it’s easier to manage the farm on the roof in part because it’s a smaller space. My “field walks” are a quick jaunt on the metal pathways between our 16 roofs. I find I am more motivated to ensure all our space is well-used because of our small acreage. We have a permanent bed system and can manage each roof differently if needed. They are all on separate irrigation valves. Each bed can be turned over one at a time when a crop is done.

I love the challenge of filling as much space as we can with plants. We have perennials and vines spilling off the sides of the building. I interplant where I can and where it makes sense. Under larger perennials I recently planted alpine strawberries and this past fall I planted anemones between our baby geranium plants. I’ve also decreased our pathway space to 12 inches (sometimes to the detriment of easy harvests) but it means increased production. I’ve increased the amount of plants we plant in any given bed. Our bed tops are 36 inches and we most often plant four rows at 8 inches between plants, with some exceptions.

rooftop-farming-the-riseThe view from the rooftop in Berkeley. Photo by Nicola Parisi.


We have a beautiful and robust line of hedgerows on every roof that wraps around the growing beds. They attract hummingbirds and pollinators to the roof, but the roots of these plants have gotten so large they grow into our production beds and can be a pain to remove. We’ve taken to root pruning all the hedgerows before we plant just to be sure our crops won’t have to compete for root space and water. 

We grow one annual crop that needs trellising — sweet peas. We devised a system to attach the trellis to the rooftop, but we have to duck when harvesting so we do not run into the wires. We get little to no chill hours on the roof and because the soil is shallow it warms up quickly. Our fall planted corms need to be covered with a shade cloth so they don’t get too hot. The shade cloth is also attached to the rooftop and makes harvest a bit more challenging as well.

One of the biggest challenges is lack of a greenhouse on the rooftop. We likely could build one that attaches to the building but that will require permission from the owners and the city. So, for the past two years I have been using my greenhouse in Sunol. During the spring and summer, I go to Sunol about once a week. I don’t have to worry about watering the babies because we have automatic irrigation, but it is a lot of schlepping plants back and forth from Sunol to Berkeley and then up to the rooftop to be planted. We recently built some mini hoop houses that we’ve been using on the roof. Each one only holds five 1020 trays, but I may decide to build more. We are weighing these down (so they don’t blow off the roof) with small sand bags. 

rooftop-farming-the-riseRow cover on the flowers with the Berkeley skyline in the background. Photo by Nicola Parisi.


The elevator is essential to our success. Sometimes I dream about creating a pulley system to lower things down to our customers waiting below. But for now, the elevator functions as a sort of farm road. We use it every day to bring up tools and take our harvest down to our pack-out area and cooler in the basement and eventually out to our customers. Going up and down in the elevator can feel cumbersome, especially when we have to wait for the elevator, or worse when it breaks down. A separate freight elevator for farm use would be ideal but we make do. 

At full capacity the building houses around 250 residents, primarily University of California Berkeley students. All residents have access to the farm and for the most part people come up and enjoy it. Once in a while we find beer bottles and footprints in our growing beds. 

We also have to be careful not to track dirt into the roof pathways where everyone walks. The pathways are made of metal grates where dirt and water can fall through to the people walking below. The most cumbersome of tasks, and one that is hard to do without dropping bits of plants and dirt, is getting plant debris off the roof. When a crop is done, we pull it out, take it down the elevator, pile it into our van and drive it to the city compost.

rooftop-farming-the-riseStrawflower harvest with Mt Tamalpais in the background.


Overall, the rooftop farm has been a huge success. I love farming in the city where I grew up. My commute to work is six blocks. I enjoy having visitors and customers come see the farm, and I love teaching about what we do. Last year we started a pop-up farmstand that we will likely continue this year. It has been a fun way to invite people (at a safe distance) to check out what we’re doing and support us at the same time. I hope to continue to teach and help advocate for urban farms and rooftop farms. I am also considering starting a non-profit arm of the business to do more educational training for youth.

I would love to see more rooftop farms. Rooftops are spaces that are otherwise not being used, and my farm is proof you can make a living on a small acreage on top of a building. The cost of building a rooftop farm certainly can be prohibitive, but I was lucky enough to have found a building whose owners built the farm and covered its cost. I am hoping that more people will see the benefits of rooftop farming and urban agriculture so that we can build more resilient cities where flowers and food abound.


Joanna Letz runs Bluma Farm in Berkeley, California. You can follow Joanna on instagram at @blumafarm, or reach her at [email protected]. She also offers one-on-one farm consultations.