Vegetable Crop Rotation

Pat Allen, EMG

Published in the Stanly Quarterly Gardener December 2020

Have you ever wondered about crop rotation and why it is important? Me too. My curiosity had been sparked and I wanted to know what it is, and why I should do it. I didn’t really need one more thing to add to my already long list of garden chores, but I wanted to know more.

The notion of crop rotation has been around for a long time but I thought it was for the big farmers with hundreds of acres, not me with a few raised beds. After all, I only grow food for me and my husband along with a few treats for our critters: apples for the horses, lettuce, spinach and other salad greens for us as well as the chickens and goats.

I started my vegetable gardening research here and found an entire section devoted to vegetables. Eureka! Here is a great starting point: https://gardening.ces.ncsu.edu/gardening-plants/vegetables-2/.

I checked off the things I had already done: selected the site, built my raised beds, had a soil test, amended the soil, learned more about our favorite cool-season and warm-season vegetables.

There are six things you need to know about rotating vegetable crops.

  1. Annual vegetables thrive at different times of the year. Warm-season annuals prefer hot, humid days, whereas cool-season annuals prefer not so hot, shorter daylight periods, and cooler temperatures to flourish.
  2. Annual Vegetables fit into classification families. Group annual vegetables into classification families because related plants tend to have the same nutrient needs and become susceptible to the same plant diseases, pests, and weeds. But, do not put them in the same plot for more than one year.
  3. Plants have different nutrient needs. Even though plants require nutrients to thrive; different plants use nutrients in a variety of ways. For instance, some need more water and nitrogen than others and, conversely, many plants add nitrogen to the soil. Nutrient balance and microbial health are key elements in feeding your vegetables.
  4. Vegetable roots grow in different patterns and depths. Some veggies have shallow (yet wide) roots (12 to 18 inches), many grow to a medium depth of 18 to 24 inches, still others have deep roots of 24 to more than 36 inches.
  5.  Rotating crops reduces the likelihood of pests, pathogens, and weeds surviving in the soil. Crop rotation works best when the garden can be divided into four or more plots. The following Table illustrates a sample four-year crop rotation plan for a garden with four plots growing vegetables from four different plant families.
Sample Crop Rotation Plan Table
 Plot 1Plot 2Plot 3Plot 4
Year 1ABCD
Year 2BCDA
Year 3CDAB
Year 4DABC

Notice how each plot is rotated. Not once in four years is the same Family planted in the same plot. Each plant Family is planted in a different plot every year for four years. The good news is that this model is flexible: If you grow five crops, then the rotation is for five years; for six crops, the rotation is for six years.

  1. Write it down. Garden Journaling is one of the most important things you can do in gardening. Having a garden plan makes it easier to decide what seeds or transplants to purchase, how many will be needed, and when they will be needed. Keeping a garden journal with previous garden plans is a good way to record what worked and what didn’t.  Journaling is beyond the scope of this article but NC State Extension has an excellent article about it. https://content.ces.ncsu.edu/extension-gardener-handbook/appendix-a-garden-journaling

Let’s get into more detail and find out what these things REALLY mean.

  1. Annual vegetables thrive at different times of the year.

No need to wait until spring to start your garden planning. Start with the current season. Grow what you like to eat in early winter and through the whole year.

On the surface, it looks like there are four seasons: Spring, Summer, Summer, and Fall. However, upon closer inspection, you’ll notice that there are actually many more because of the transition between seasons. As Dr. Lucy Bradley states, “Plant cool-season crops early [spring] and warm season crops in late spring.”

The transition between seasons not only adds more food to your plate but by having longer growing seasons, your plants keep feeding your soil. Because of these transitional seasons, it’s important to plan your crops for the whole year – not just for one season. Keep your plot covered with either vegetables or cover crops to protect your soil and its nutrients from leaching away.

Winter. Crops considered cold hardy (Brassicaceaes: collards, kale, turnip greens, spinach, and Swiss chard) planted in fall may live through the winter or may go dormant for a period in the winter and flourish again in early spring. A cold frame or frost cloth can help protect and extend this growing season. Many of these plants bolt quickly in the spring.

Spring. Cool-season annuals are cold-hardy plants that thrive in the early spring and fall when temperatures fall below 70°F. To get a jumpstart on the spring season, use a cold frame, low tunnel, or frost cloth.

Late Spring. Warm-season annuals are frost-sensitive crops that grow well in the late spring when temperatures are above 70°F and soils have warmed up.

Summer. As temperatures rise, cool-season crops bolt and become bitter. Use shade cloth or taller crops to provide shade and extend cool-season crops into summer. Warm-season crops planted in late spring usually grow through summer until the first frost.

Late summer is the time to plant cool-season annuals for a fall harvest.

Fall. Cool-season annuals that are well-established grow through cold temperatures and sometimes even moderate freezing.

For more details about planting seasons, see the following link:
https://content.ces.ncsu.edu/extension-gardener-handbook/16-vegetable-gardening#gardeningthrough

For information on specific vegetables, see the following link:       https://extensiongardener.ces.ncsu.edu/wp-content/uploads/2016/07/NC-Vegetable-Planting-Guide-1.pdf?fwd=no

  1. Annual vegetables fit into classification families.

After you’ve selected the vegetables you want to grow, group them into classification families.

The Vegetable Identification Table below describes the edible parts of many vegetables. In addition to the typical headings, I’ve added a column for “Harvested Part” because vegetables can be grouped by edible parts of the plants. Most plants fall into more than one category. For this reason, I found it difficult to use this classification as a way of family groupings, as some researchers recommend.

  • Bulbs usually grow just below the surface and produce fleshy, leafy shoots: fennel, garlic, leek, onion
  • Flowers of many plants are edible: artichoke, cauliflower, broccoli, squash flowers.
  • Fruits are fleshy and contain seed: melon, cucumber, eggplants, melon, pumpkin squash, tomato
  • Fungi are usually known as mushrooms: button white, oyster, shiitake,
  • Leaves are edible: bok choy, cabbage, lettuce, radicchio, spinach,
  • Roots are usually long or round-shaped taproot: carrot, parsnip radish, turnip
  • Seeds grow in pods which are sometime eaten with the seed: bean, pea, snow pea, sweet corn
  • Stems are the stalks of plants when the stalk is the main part of the vegetable: asparagus, celery, kohlrabi
  • Tubers grow underground on the root of the plant: Jerusalem artichoke, potato, yam

This information is anecdotal but offers an experiential point of view:

The Vegetable Identification Table lists the Botanical Family, the commonly known term for a collection of plants, whereas the Scientific Name is the Official name. Common Names are what we call the individual plants. The Harvested Parts category is a new concept that I hadn’t thought of before but, turns out to be a relatively common term in the chef world. Cool…learned something else new.

Vegetable Identification Table
Botanical FamilyScientific NameCommon NameHarvested Part
Lily, Onion, or Allium FamilyAlliaceae / Liliaceaechives, garlic, leek, onion, and shallotsroot, bulbs,
Goosefoot/Beetroot Family.Amaranthaceae (Chenopodiaceae)amaranth, beetroot, beets, goosefoot, lamb’s quarters, quinoa, spinach, sugar beet, and Siss chardroots, tubers, leaves
Aster, Composite or Sunflower FamilyCompositae / Asteraceae Artemisia, chamomile, chrysanthemum, chicory/endive, dandelion, globe artichoke, Jerusalem artichoke, lettuce, marigolds, safflower, salsify, sunflower, and tarragonleaves, flowers, roots
Morning Glory FamilyConvolvulaceaesweet potatotuber, leaves, shoots
Crucifer/Cabbage/Mustard familysCruciferae / Brassicaceae bok choy, broccoli, Brussel sprouts, cabbage, cauliflower, Chinese cabbage, collard greens, cress, horseradish, kale, kohlrabi, mustard, radish, rutabaga, turnip, wasabi, and watercressleaves, stems, flower heads and buds, roots
Gourd or Squash FamilyCucurbitaceasummer squash, watermelon, winter squash, and zucchinifruits, flowers, roots
Pea or Legume Family known as ‘nitrogen fixers’, adds nitrogen to the soilFabaceaealfalfa, bean, clover, lentil, lupin, pea, peanut, and soybeanseeds, seed pods
herbsLamiaceaebasil, catnip, hyssop, lavender, marjoram, white horehound, lemon balm, oregano, rosemary, savory, sage, and thymeleaves, stems
Grass FamilyPocaceae or Gramineaebarley, corn/maize (grow after carrots), mallet, oats, rice, rye, sorghum, sugarcane, sweet corn, and wheatseeds
Nightshade/Potato FamilySolanaceaechile pepper, eggplant, paprika, pepper, potato, and tomatillofruits, tubers
Parsley or Carrot FamilyUmbelliferae / Apiaceae caraway, carrot, celeriac, celery, cilantro, coriander/cilantro, cumin, dill (don’t plant with carrots or tomatoes), fennel (always plant alone), parsley, parsnip, and Queen Anne’s lacefruits, stems, flowers, root
https://content.ces.ncsu.edu/home-vegetable-gardening-a-quick-reference-guide

Before discovering this table, I never realized the relationships that exist among certain vegetables. Now that I know more about the botanical families and the vegetables in their respective groups, I’m beginning to see why pests, pathogens, and weeds tend to be attracted to certain vegetables.

In crop rotation, pests and diseases build up in the soil near host plants. Reduce this possibility by rotating crops every on a planned basis. For example, “squash often get vine borers. Broccoli, collards, and cabbage get cabbage loppers, and melons are prone to fungus and other diseases.” Knowing the common pests, diseases, and weeds among Families not only helps you identify the culprit but helps you avoid trouble in the first place – by rotating your vegetables.

For more details on pest management, https://content.ces.ncsu.edu/extension-gardener-handbook/8-integrated-pest-management-ipm#section_heading_5177

  1. Plants have different nutrient needs

Some vegetables like Brassicaceaes (Cabbage/Mustard Families) are heavy feeders that deplete nitrogen more quickly than light feeders like root crops (Umbrelliferae) and the Onion Family (Alliumn or Illiaceae). On the other hand, the Pea and Bean Families (Leguminosae and Fabaceae) return nitrogen to the soil. Known as ‘nitrogen fixers’, cover crops like clover, hairy vetch, buckwheat are also included in this category.

The following bullet points list a few vegetables along with their feeding habits.

●       Heavy Feeders: asparagus, beets, broccoli, brussels sprouts, cabbage, cantaloupe/honeydew, cauliflower, celery/celeriac, corn, cucumber, eggplant, endive, kohlrabi, leeks, lettuce, melons, okra, onions, parsley, peppers, pumpkin, rhubarb, spinach, squash, sunflower, tomatoes, turnips
●       Medium Feeders: artichoke, basil, beans, bok choy, brussels sprouts, cabbage, cauliflower greens, cilantro, cucumbers, eggplant, garlic, kale, lettuce, okra, peppers (small-fruited), potatoes, pumpkin, radish, rutabaga, scallions, spinach, squash, sweet corn, swiss chard, tomatoes, watermelon, zucchini
●       Light Feeders (aka ‘Nitrogen Fixers’): arugula, beans, beets, bulbs, carrot, chard, chicory, clover, garlic collard greens, endive, escarole, fava beans, herbs (most kinds), herbs, kale, leeks, mustard, onions, parsnip, peas, pepper, potato radish, rutabaga, shallot, Swiss chard, turnip
Gardens benefit from well-balanced soil but keeping it balanced isn’t easy. Understanding the difference between heavy feeders and light feeders makes sense and encourages me to want to be more aware of nurturing my soil.

 

  1. Vegetable roots grow in different patterns and depths

As mentioned earlier, some veggies have shallow (yet wide) roots (12 to 18 inches), many grow to a medium depth of 18 to 24 inches, still others have deep roots of 24 to more than 36 inches. The benefit of varying root depths is that they interweave among each other taking advantage of nutrients at different soil levels. This means your soil has more balanced nutrient availability for a variety of plants.

Plants with shallow roots extract water and nitrogen near the surface whereas, other crops explore soil at lower levels. Following a shallow-rooted crop, many farmers will plant deeper-rooted crops to recover nutrients unused by the shallow feeders.

The Vegetable Root Depth table is a sample list of some of the plants. Organic farmers practice this theory and use this literature as a guideline.

Vegetable Root Depths
Shallow Rooting (12 to 18 inches)Medium Rooting (18 to 24 inches)Deep Rooting (24 to 36 inches)
arugulabeans (fava, pole, snap)artichokes
basilbeetsasparagus
blueberriescantaloupesbeans (lima)
bok choycarrotsburdock root (gobo)
broccolichardcardoon
brussels sproutscucumbersokra
cabbagedaikonparsnips
cauliflowereggplantpumpkins
celeriacmuskmelonsrhubarb
celerypeas (shelling. snap snow)squash (winter)
chivespeppers (hot & sweet)sweet potatoes
cilantrorosemarytomatoes
collard greensrutabagaswatermelons
cornsage 
endivesquash (summer) 
fennelturnips 
  1. Rotating crops reduces the likelihood of pests, pathogens, and weeds surviving in the soil.

Crop rotation works best when the garden can be divided into four or more plots. As mentioned earlier, the Sample Crop Rotation Plan Table illustrates a four-year crop rotation plan for a garden with four plots growing vegetables in four different plant Families. Each Family has different nutrient needs and has roots that grow to different depths and widths. These similarities, if repeated year after year, create voids/weaknesses in the soil that attracts pests and pathogens.

Sample Crop Rotation Plan Table
 Plot 1Plot 2Plot 3Plot 4
Year 1ABCD
Year 2BCDA
Year 3CDAB
Year 4DABC

Notice that each plot is changed every year. Botanical Family A has been bolded and colored red to illustrate how it moves through the rotation. It takes four years for each crop to make the rounds before it’s returned to its beginning plot. Since each plot receives a different crop, the life-cycles of pests, pathogens, and weeds are broken and have a reduced chance of impacting the soil.

Sample Four-Year Rotation for Year-Round Gardening Table
 Plot 1Plot 2Plot 3Plot 4
Year 1Pea FamilyTomato & Potato FamilyRoot Crops/Onion FamilyLettuce Family
Year 2Tomato & Potato FamilyRoot Crops/Onion FamilyLettuce FamilyPea Family
Year 3Root Crops/Onion FamilyLettuce FamilyPea FamilyTomato & Potato Family
Year 4Lettuce FamilyPea FamilyTomato & Potato FamilyRoot Crops/Onion Family

As with the Sample Crop Rotation Plan Table, crop A has been marked red, so has the Sample Four-Year Rotation for Year-Round Gardening Table so you can see how the Pea Family travels through the rotation plan.

  1. Write it down!

Part of garden planning is reviewing what has worked in the past and what didn’t, so past mistakes can be avoided in the future. Please read the Extension chapter on Garden Journaling; it is excellent and will give you many worthwhile ideas.

For now, you know how to plan your crop rotation:

  • Identify the plats you want to grow,
  • Put them in their Classification Families,
  • Be aware of which plants are heavy feeders, light feeders, and nitrogen fixers,
  • Be aware of your crop root growth habits so you can plant interweaving plants and maximize your crop nutrients, thereby reducing pests, pathogens and weed pressures,
  • You’ve planned for a year so you already have a plan and know what you’ll plant where next season, and
  • You’re written everything down so you remember (have a record) of what worked and what didn’t.

Additional Resources

*A hard copy only costs $24.00 and is well worth the money, in my opinion.

Center for Environmental Farming Systems (CEFS) offers a free booklet about crop rotations on organic farms. It can be found here: http://carolinafarmstewards.org/wp-content/uploads/2012/12/7-CEFS-Crop-Rotation-on-Organic-Farms.pdf

The Vegetable Garden Rotation, a common sense guide is the article that started my quest. It’s short and to the point but made me ask more questions once I got to thinking about rotating.
https://www.co.thurston.wa.us/health/ehcsg/pdf/CSG_VegeGarden_sglpg.pdf