Crop rotations and crop planning

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Figure 15: Start thinking about crop rotation by imagining you have three beds (A, B, and C), and you plant the same crop in each bed every year.

What is crop rotation and why would someone do it?
Imagine a simplified urban farm with three beds: A, B, and C. In bed A, you plant tomatoes, in bed B you plant collard greens, and in bed C you plant peas (Figure 15). Next year, out of habit, you do the same thing. And the same thing the year after that.

If you keep doing this year after year, eventually you run the risk of having diseases specific to each crop build up in the soil. But because each of these crops come from different plant families, many diseases that infect tomatoes do not infect collard greens, and vice versa. So a good strategy to prevent disease is to rotate your crops from bed to bed every year (Figure 16). This means that tomato disease agents left in the soil after year 1 will have no suitable host in years 2 and 3, and will be more likely to die off before tomatoes are planted again in year 4. This is crop rotation.

By rotating your crops like this, you not only reduce the risk of disease, you also take advantage of the fact that your peas are a nitrogen-fixing legume. Every year they pull some nitrogen out of the air and leave it in the soil. If you grow a crop that needs a lot of nitrogen, like collard greens, in a bed that had peas the previous year, the collard greens will benefit from the nitrogen that the peas left in the soil.

An advanced crop rotation plan will balance many factors like diseases and nutrient needs. To learn more, a good book to start with is Crop Rotation on Organic Farms, by Charles L. Mohler and Sue Ellen Johnson. A free digital version is available online from Sustainable Agriculture Research and Education: https://www.sare.org

Figure 16: An example of a simple crop rotation design. Every year you shift which crop you plant in which bed. This helps break up disease and pest life-cycles, and helps replenish the nutrients in the soil.

Less information is available about how to apply the principles of crop rotation to indoor production, but understanding what pests and diseases affect which families of crops is still valuable. The next topic, crop planning, is crucial for both outdoor and indoor farms.

What is crop planning and why would someone do it?
In the Marketing chapter, you’ll hear from Ginger and Kim that they encourage farmers to plan where you will sell your crops before you even plant them.

So let’s imagine you plan to sell vegetables through a Community Supported Agriculture (CSA) weekly farm share. You want to deliver to thirty customers a box of at least three different kinds of fresh, delicious vegetables every Friday, from June through September. You can then plan backwards from this goal to figure out what you need to plant when, and how much of it! This is crop planning.

Seed labels and catalogs can be a helpful resource in this process. Some things to consider include

• the average frost dates in the spring and fall for your location (if growing outdoors),
• what crops are best suited to the cool parts of the season (lettuces, brassicas, and most herbs, for example) and what crops need the heat of summer to thrive (tomatoes, peppers, okra, and sweet potatoes, for example),
• which crops can be direct-seeded and which will need to be started earlier as transplants,
• how much space different crops take up and roughly how much produce you can expect each plant or square foot to yield,
• roughly how much time different crops need to produce their edible portion (“days to maturity” on a seed packet) and whether it will be possible to “succession plant” multiple crops in one spot at different times in the growing season.

Succession planting has two meanings:

1. Growing multiple crops in the same space, but separated by time. For example, you could plant spinach early in the spring, pick spinach leaves throughout the spring, pull up or till under the spinach in late spring, and plant tomato transplants in the spinach’s place. The spinach cannot tolerate the heat of summer and will bolt and the tomatoes need the heat of the summer and cannot be planted outdoors before the last risk of frost, so you grow them in the same place in succession to maximize how much food you can produce per square foot.
   
   
2. Planting the same crop in different places, but on multiple planting dates, to stagger their maturity and harvest dates. For example, you could plant one row of cilantro every week in April, so that you would be able to cut big beautiful bunches of cilantro from each row in succession for your weekly farmers market in May.

Crop planning is important not just for CSA farmers, but also for other market outlets such as farmers' markets or contract growing. For example, farms that are associated with schools need to carefully plan how to deliver food during the school year, which is not the traditional growing season in the US! Tufts University offers a very helpful crop planning lesson module at https://nesfp.org/resources/crop-planning-module.

For crop planning in Maryland's climate, this color-coded planting calendar is a fantastic resource: https://go.umd.edu/plantingcalendar.

Urban farms that use high tunnels to extend the growing season and farms that use hydroponic methods to grow indoors will also find crop planning crucial.

In high tunnels or hoop houses, crop planning will help you efficiently use your limited sheltered space and work with the seasons. In particular, if you are trying to grow year-round, note that in northern and Mid-Atlantic climates even in a high tunnel plant growth slows dramatically in the winter. Farmers I’ve known who have successfully used their high tunnels through the winter have been careful to get their winter crops planted at just the right time in the fall so that the crop will have time to grow to the early edible stage before the cold sets in, and the high tunnel will then almost act as a stasis field through the coldest part of the winter—keeping the crops alive and fresh but not growing much more, so they can be harvested in the winter when few other sources of local produce are available. Elliot Coleman’s books Four Season Harvest and The Winter Harvest Handbook are the foundational texts on season extension in cold climates. University of Delaware Extension has researched recommended planting dates for high tunnels in our climate region: https://go.umd.edu/hightunnelplanting

In hydroponic and aquaponic urban farming, efficiently producing the most food per unit of space and time is crucial for covering the start-up, maintenance, and utility costs of a hydroponic system. In this case, the succession planting part of crop planning is particularly important. Successful hydroponic farmers spend time figuring out how long it will take their system to produce different kinds of crops, and how to stagger planting dates to enable them to harvest continuously.

For example, as an aquaponic urban farmer you might start a small amount of several different kinds of greens and herbs as plugs every week, so you can plant a few flats of new plugs in your float tank every couple of weeks and harvest a few flats every week for a farmers market (succession planting type 2 above).

Alternatively, a hydroponic or aquaponic grower selling a large amount of lettuce to a cafeteria every month might plant their entire growing space at one time on a date calculated so that the lettuce will be mature in time for the contracted delivery date. However, such a grower would need a large amount of space for plug production so they could be ready to plant again immediately after harvest (succession planting type 1 above).

In summary, crop rotation is an important strategy to manage production risks such as pest and disease pressure and nutrient deficiency. Crop planning is an important strategy to manage market risks, enabling you to plan what and when you grow with your end market in mind, and enabling you to take advantage of the opportunities offered by the lower supply of fresh, local vegetables in the early spring, late fall, and winter.

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