The great thing about gardening with annual vegetables is that it’s very cheap to get started and the plants grow very quickly, so gardeners get to see (and taste) the results of their efforts in a very short time.
Some gardeners get discouraged after a few years of successful vegie gardening when things start changing. Plants may seem to not grow as well anymore or appear to be less productive. Sometimes it’s worse – plants may even die outright from strange diseases. Gardeners may come to the conclusion that gardening is difficult and their initial success was just beginners luck!
Not feeding the garden with fertilizer (at the start of spring and autumn) is a common beginners mistake, and that’s easily fixed by feeding the garden. But what if you do feed your vegetable garden and it’s still getting less productive?
Quite often the reason why accomplished vegetable gardeners run into major issues with productivity and diseases in their vegetable garden is because they’re not practising crop rotation. They may make the mistake of designing a permanent garden patch for tomatoes, or putting aside a favourite spot in the garden for growing broccoli, and then plant the same vegetables in the same location, year after year, which is a sure recipe for garden problems!
What is Crop Rotation?
Crop rotation is the practise of planting a particular crop in a different spot in the garden each year, moving the planting location from year to year so the any garden location will never grows the same type of plant for longer than a year.
The way that this is accomplished is that each type of vegetable is planted in separate garden beds in a certain sequence or order, and then each year the planting location for each type of vegetable is moved to the next spot in the sequence or order. This systematic approach ensures, for example, that tomatoes will not be grown in the same garden bed for two years in a row.
Let’s have a look at an example to see how this system works.
In this example there are three garden beds.
In the first year we plant tomatoes in the first garden bed, in the second year we plant them in the second garden bed, in the third year we plant them in the third garden bed.
What happens after three years? In the fourth year we go back to the start and plant the tomatoes back in the first garden bed once again. We keep repeating the sequence each year, it’s that simple.
What happens to the other vegetables in the other beds? They all move along one spot to the next bed too, as shown in the diagram.
Benefits of Crop Rotation
One of the primary aims of a vegetable gardener is to grow food, the more the better, and crop rotation is a system that will increase yields, and maintain high yields over the long term.
The practise of crop rotation maximises yields by:
Preventing the depletion of soil nutrients – certain vegetables will use more of some nutrients and less of others. If you grow the same vegetables in the same spot over and over, they will deplete the most used nutrients for that plant very quickly.
Reducing pests and diseases – if the same sorts of vegetables are grown in the same location continuously, pests that prefer those vegetables will gather there. If you keep planting more food for the pests right where they live, their populations will grow rapidly and you’ll have a major pest problem on your hands. Move their food and it breaks their breeding cycle. Also, pathogens that cause root diseases can build up in the soil, reinfecting the plants they favour if you keep replanting in the same location. Another benefit of naturally reducing pests and diseases is a reduction in chemical use in the garden (assuming you use garden chemicals).
Keeping soil healthy – certain plants used in crop rotation actually add nutrients back into the soil, maintaining nutrient levels to keep the soil healthy to support plant growth.
How to Rotate Crops
At this point you’re probably wondering how do we rotate our crops?
There are many possible systems of grouping plants together to rotate them. We can’t simply sort our plants by their common names, such as tomatoes, capsicums, chillies, eggplants, potatoes and rotate them that way. Why? The plants just mentioned are in fact all from the same family (the Solanaceae or Nightshade family) and share common diseases!
We need ways of grouping the various types of vegetables to avoid nutrient depletion and the build-up of pests and diseases.
In this article we’ll look at two systems of crop rotation:
- Crop Rotation by Plant Family
- Crop Rotation by Plant Nutrient Demands
Let us have a look at each of these systems and how we can use them in our gardens.
Crop Rotation by Plant Family
Botanist classify plants according to a complex hierarchical system, the details of which doesn’t really concern us. Within this system of classifying plants we find that plants are classified by Family, then Genus, then Species.
All plants belong to the same family will be related and have common traits and properties.
A tomato is classified as Family: Solanaceae, Genus: Solanum, Species: Solanum lycopersicum
A potato is classified as Family: Solanaceae, Genus: Solanum, Species: Solanum tuberosum
As we can see from this example, tomatoes and potatoes not only belong to the same family, but the same genus, so they’re quite closely related, which means they will have things in common and as a consequence be affected by similar diseases.
By grouping plants by Family, we can rotate whole plant families from bed to bed and eliminate the problem of spreading diseases through the soil that are common to particular families.
The rules for Crop Rotation by Plant Family are quite simple:
- Make sure you don’t plant a family member in the same spot 3 years in a row.
- Don’t follow with same plant or plant from same family (i.e. brassicas)
- Don’t follow root crops with root crops (i.e. carrots with beetroot).
For example , in one bed, plant beans (legume family), the next year plant potatoes (tomato family), the third year plant cucumbers (squash family). After that, you can start over with another legume-family vegetable or start another rotation series.
Crop Grouping for Rotation to Control Soil-borne Diseases
|Group A||Group B||Group C||Group D||Group E||Group F|
|Sweet corn||All beans|
With six beds, we can plant each group in each bed (Group A in bed 1, Group B in bed 2, etc.) and then rotate to the next bed the next year (Group A in bed 2, Group B in bed 3, etc.).
Keeping in mind that some vegetables only grow in the warmer or cooler seasons, we wouldn’t be rotating all groups listed in the table at the same time, as this won’t make the most efficient use of the beds. For more efficient use of garden space we can plant two beds simultaneously with the same group, say if it’s the tomato family (Group C), we can plant beds 1 and 3, then next year beds 2 and 4, followed by beds 3 and 6 – this will give a 3 year cycle.
Typically during the cooler seasons there are less plants that can be grown so we can double up on planting groups. Say we’re looking at the brassica family (Group B), we can plant beds 1 and 3, then next year beds 2 and 4, followed by beds 3 and 6 – this will give a 3 year cycle once again.
Refer to the planting calendars for when to sow the vegetables from each group. The planting calendars indicate when to sow the vegetable seeds. The seedlings are usually transplanted into the garden about 4 weeks later if they are large enough to transplant.
Crop Rotation by Plant Nutrient Demands
Some plants need more nutrients (food or fertiliser) than others, and some plants actually add nutrients back to the soil. If we always put plants that take up huge amounts of nutrients in the same location in the garden, they will quickly deplete the soil and our productivity will decline. By rotating plants with different nutrient demands on the soil, we can maintain soil fertility and maximize productivity.
In the book “How to Grow More Vegetables” by John Jeavons, we are introduced to a system of crop rotation that categorizes plants as Heavy Feeders, Heavy Givers and Light Feeders.
We can describe each of the categories as follows:
Heavy feeders – most vegetables fall into this category, these are very hungry plants that take a lot of nutrients out from the soil. They require a lot of nitrogen in particular, which plants use for green leafy growth.
Heavy Givers – these are nitrogen-fixing plants that can take nitrogen from the air and convert it into a form usable by plants. They return nitrogen to the soil, maintaining soil fertility.
Light Feeders – which includes all root crops, only use a small amount of nutrients, giving the soil a rest and a chance to recover before the heavy feeders are planted once again.
To use this system, in any garden bed, rotate the plant groups in the order of Heavy feeders -> Heavy givers -> Light feeders.
Here is a list of vegetable nutrient requirements for each of the three categories:
- Heavy Feeders – Asparagus, Beet, Broccoli, Brussels sprouts, Cabbage, Cantaloupe, Cauliflower, Celery, Collard, Corn (Sweet), Eggplant, Endive, Kale, Kohlrabi, Lettuce, Okra, Parsley, Pepper, Potato, Pumpkin, Radish, Rhubarb, Spinach, Squash (Summer), Strawberry, Sunflower, Tomato, Watermelon
- Light Feeders – Carrot, Garlic, Leek, Mustard Greens, Onion, Parsnip, Rutabaga, Shallot, Sweet Potato, Swiss Chard
- Heavy Givers – Alfalfa, Beans, Clover, Peas, Peanut, Soybeans
(Source: “Arizona Master Gardener Manual” by Arizona Cooperative Extension, College of Agriculture, The University of Arizona)
The heavy givers are soil building plants and this category includes all nitrogen-fixing plants, of which there are many, much more than are listed above.
There’s also another option possible with this system, a fourth stage that allows additional soil building, and there are two ways to do it:
- Growing a green manure crop – a combination of cereal grasses and legumes, which is grown then chopped down or dug in to add bulk organic matter and nitrogen back into the soil, which partly duplicates the ‘heavy giver’ stage
- Resting the garden bed – by adding compost to the soil, then covering it with a straw mulch and letting it sit for a season
(Note: this is a no-dig gardening method for preparing the soil between seasons – fertilizer is also added under the mulch when we use this method, remember to always fertilize the garden at the start of spring and autumn whichever way we garden!).
If you’re curious about green manure crops, there are both warm season and cool season plants used for this purpose to increase soil fertility.
- Warm Season green manures include buckwheat, cowpea, French white millet, Japanese millet, lablab, mung bean, soybean.
- Cool Season green manures include broad bean, fenugreek, lupins, mustard, oats, subclover, vetch.
Depending on your climate and geographical location the plant varieties used as green manure may vary somewhat, but they all work the same way.
Small Space Crop Rotation
What do we do if we only have a small garden, or very few garden beds, how can crop rotation be used in such instances?
It’s important to be able to ‘think outside the square’ and come up with creative solutions when faced with such problems.
- Use different corners of a wide square garden bed to rotate around and plant perennials in the middle.
- Use opposite sides of a long rectangular garden bed to rotate around and divide it into multiple beds using a section of perennial plants as a divider.
- Rotate from your garden beds into containers to add an extra rotation location.
Practising crop rotation of annual vegetables is a sound way to minimize pests and diseases, increase yields and keep the soil healthy and fertile in the process. By creatively using garden pots and other containers we can create the equivalent of an extra bed to rotate through and create extra growing space in small-space gardening situation. Unless we’re growing vegetables in a food forest environment where annual plants are scattered randomly through a predominately perennial gardening system, then annual vegetable crop rotation is a must for all produce gardeners!