The importance of rotating crops

Are your beetroot plants all leaf and no beet, your carrots forked, and your tomato plants are succumbing to a mysterious disease?  Might not be their fault.  It might be because you are not rotating your crops. In this blog post, after talking about the 'what and why', we have a look at a simple crop rotation plan that can make your food-garden much more successful.

Jennie and Russell's inspiring food-garden at Sandfly

Isn't crop rotation something only professional farmers do?

Many food-gardeners struggle with the concept of crop rotation.  It is something professional farmers do if they are wise, but it is equally important in our veggie-gardens.

Rotating vegetable crops is important in all situations if you want good results, not just this season, but every season.  

Rotating crops is important in every food garden because ...

• It does not allow soil-borne pests and viruses associated with a crop to get established.

• You avoid depleting your soil of nutrients that a particular crop or plant family uses.

• It allows you to plant crops in a certain succession that benefits plants that come after them.

• It makes it easy to remember where to apply and not apply compost and organic fertilisers.

What crops should I rotate?

Crop rotation is used for annual vegetables that are grown in established garden beds that are intended for a succession of vegetables over the years. It is not used for Asparagus, berries, fruit trees and other permanent plants.

Okay, so let's see, in this bed here - I wrote it down in my garden-diary - I had capsicums two years ago, tomatoes last year, and now I have potatoes, so I have rotated my crops, haven't I?

And over here I had cauliflowers, then radish and now I am trying to grow some swedes, but they are not doing so well.

Crop rotation means not having the same plants in the same spot for more than a year, but there is more to it. It also means not having other members of the same plant family in that same spot either. 

If you have capsicums, then tomatoes, then potatoes in a garden bed, then you have three members of the same plant family in that bed for three years: not a good idea! Even though they look quite different, capsicums, tomatoes and potatoes are members of the same plant family. 

The story for the other bed is the same: cauliflower, radish and Swedes look very different, but they are all part of one and the same plant family.

Pests and diseases affecting one variety get a real opportunity to establish themselves because they are likely to feel equally at home with other members of the same plant family.

Here is a list of the most common annual food-garden plants and what family they are part of.  It is worth realising which vegetables belong to the same family:

Known as	Part of this family are
Alliums (Amaryllidaceae family)	Garlic, Leek, Onions, Shallots, Asparagus
Root crops Apiaceae family	Carrot, Parsnip, Celery, Celeriac
Asteraceae family	Endive, Lettuce, Yacon
Brassicas (Brassicaceae family)	Broccoli, Cauliflower,  Cabbage, Kale, Brussels Sprouts, Kohlrabi, Radish, Rocket, Swede, Turnip, Wong Bok, Bok choy, Pak Choi and Tatsoi
Chenopodiaceae family	Beetroot, English Spinach, Silverbeet, Chard
Cucurbits (Cucurbitaceae family)	Cucumber, Pumpkin, Squash, Zucchini, Melons
Legumes (Leguminosae family)	Broadbean, Bean, Pea
Poaceae family	Sweetcorn
Solanums (Solanaceae family)	Capsicum, Chilli, Eggplant, Potato, Tomato, Oca

This table it interesting because it holds some surprises: not all root vegetables are part of the same plant family and some brassicas don't look at all like brassicas. This is good to know as plants that are part of the same plant family will in many cases share fertiliser requirements, garden pests and diseases!

The special role of Legumes

One of these plant families plays a special role in crop rotation.  Legumes have bacteria in their root nodules that take nitrogen out of the air and make it available to the host plants and subsequent crops if, when removing the old plants, you cut the stems at ground level or dig them in, rather than pulling the roots out.

Legumes, if planted just before other crops that love nitrogen, can majorly contribute to the success of that next crop. On the other hand, if the next crop does not like nitrogen, it may lead to bad results.  Tomatoes may have many leaves and few tomatoes. Beetroot may also have great looking foliage, but very small beets.  Carrots may be forked.

A simple but effective crop rotation plan

In books and on web sites you will find crop rotation plans for 3, 4, 6, up to 7-year periods.  These plans involve creating as many beds as there are years in the plan, and each year rotating crops from bed to bed.  Here is a 4-year crop rotation plan that takes most of the factors mentioned above into account.  I reckon it is realistic and workable: 

Year 1 Bed 1	In Autumn apply ample lime, dolomite and perhaps kelp, but not organic fertilizers that contain nitrogen Autumn – sow broad-beans and peas Spring – sow beans in late Spring
Year 1 Bed 2	In Autumn apply compost and the full range of organic fertilizers Autumn – plant/sow brassicas and leafy greens you want to grow over winter Spring - plant/sow brassicas, leafy greens, pumpkins, cucurbits, alliums and solanums to grow over summer
Year 1 Bed 3	In Autumn add nothing to the soil, but dig, aerate or do other things to make the soil looser and deeper. Autumn – sow parsnips, beetroot and other root crops you plan to grow over winter Spring – sow parsnips, beetroot and other root crops you plan to grow over summer
Year 1 Bed 4	In Autumn add compost, dig and aerate the soil or do other things to make the soil looser and deeper Late Winter – plant potatoes when the chance of frost is over Late Spring – tomatoes

The application of compost, manures and fertilizers can very nicely fit in with a crop rotation plan.  You can make sure that you do not apply them for crops that can not cope with too many nutrients.

This is how to start

• Autumn is a good time to start your crop rotation plan as most summer crops come to an end.

• Divide your vegetable garden into four parts.  These four beds don't have to be rectangular or adjacent to each other.  Any four areas of roundabout equal size are fine.

• Using the plan above (or a different plan created by you based on the same principles) decide on your particular crops within the plant families ('we don't like Parsnip, but eat a lot of Carrot').

• For the first year simply put the crops in your 4 beds as indicated above. The results may not be fantastic because this is only Year 1 of your plan.

• In Autumn of that season you rotate your beds: from here on put what you had in Bed 2 last year in Bed 1, what you had in Bed 3 in Bed 2, what you had in Bed 4 in Bed 3, and what you had in Bed 1 in Bed 4. 

• Rotate once more the following Autumn, and keep going with this from then on.

This is not a short-term project.  You will begin to notice benefits after a year or so when carrots no longer fork for example, but only in the fifth year, when every bed has been through the cycle once, will the full benefits become evident.

Should I rotate the crops in my hothouse? 

Every year I grow tomatoes and cucumbers in my hothouse. There are so many cool Spring seasons in Tasmania. In those weather conditions the tomato bushes in my hothouse produce ripe tomatoes a lot earlier than the ones outside. And I don't even try to grow cucumbers in my garden, as outside ones would begin to produce so much later, that I don't find it worth it.

But hold on, am I not going against my own recommendations in this blog post?

No, I don't, because instead of rotating these crops, I replace their soil!

Every winter I replace the top ten or more centimetres of soil in the beds and pots in my hothouse with fresh soil from my compost heap (mixed with coir and vermiculite and sand and complete organic fertiliser). Then I plant my tomatoes in that soil mix a few months later. The soil that I remove, I add to an outside garden bed in a spot where I won't be growing tomatoes that season.

The same applies to pots and grow bags and that favourite sheltered spot against a north-facing wall where you grow your capsicums every year: if you want to grow a crop in the same spot for more than one season, replace their soil!  You may say 'but I haven't done that and I have had no problems so far'. Well, you have been lucky!

Summarising ....

Crop rotation, in conjunction with the use of lime, compost and organic fertilisers (as indicated in the table above), is the way to have a successful vegetable garden for many decades, without that garden gradually becoming less fertile and dominated by pests. 

Happy food gardening,

Max Bee

FGG coordinator