Crop Rotation and Companion Planting in the Greenhouse

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Ever since we finished building our greenhouse, one question has been on my mind. How do you actually solve the crop rotation problem that arises in the greenhouse?

Since you tend to grow a much narrower range of plants there, often the same ones every year – tomatoes, peppers, perhaps aubergines and/or cucumbers, some of which also belong to the same plant family (nightshades) – achieving adequate rotation is quite difficult. There is then a risk that the soil quickly becomes one-sided and depleted, or that diseases take hold.

How can you solve this without having to replace the soil every year? Some people do exactly that, and it was apparently once common practice. I’ll admit that is too much effort for me.

Something I have read several times is that tomatoes can indeed be planted in the same spot for several years, as long as one of the feared diseases such as blight does not take hold.

Suitable Measures

We present several options for counteracting soil depletion and the onset of disease. Even individually they are already effective. Combine several of them and you can, with a bit of luck, keep the soil healthy for quite a while – and with it the plants.

Compost, Fertiliser and Soil Amendments

To prevent nutrient deficiency, apply a generous layer (10–15 cm) of mature compost to the soil in spring and work it in lightly. If appropriate, you can also incorporate some mature animal manure. Heavy feeders like tomatoes will be grateful.

Only fertilise with organic fertilisers. Unlike mineral fertilisers, they improve soil structure because they also feed the soil life and stimulate it to work. They are also taken up by plants much more slowly and evenly.

Rock flour (* Affiliate) and perhaps a soil activator (* Affiliate) are also worth adding. These soil amendments provide additional micro-nutrients for plants and soil life and prevent soil fatigue by enriching the soil with important minerals. They make nutrients more available to plants, meaning you need less fertiliser overall. The soil life benefits, and as a result the soil structure also improves.

Herbs and Companion Plants

To prevent diseases, harmful nematodes and other unwanted creatures in the soil and on the plants, you can also plant herbs and other companion plants between the vegetables. With richly flowering plants like tagetes and pot marigold, there is also a welcome side effect. They attract pollinating insects and lure them into the greenhouse.

French Marigolds (Tagetes):

Work outstandingly against harmful nematodes in the soil through root secretions. They are also said to help against bacterial, fungal (mycotic) and viral diseases. In addition, they attract slugs that have strayed into the greenhouse. You can find them there in the evening and collect them.

Pot Marigold (Calendula officinalis):

Also works against nematodes and wireworms, and some other pests such as the pea moth and black bean aphid. Good to know, even if you don’t necessarily grow them in the greenhouse.

Additionally it has a growth-promoting effect on almost all vegetable plants.

Antibacterial, antiviral and antifungal properties have even been demonstrated. But I cannot say whether these also work in companion planting or “only” in their role as a medicinal plant – as aqueous, oily or alcoholic extracts. You might perhaps brew a tea and pour it on after cooling. But that is pure speculation. 🤓

Basil (Ocimum basilicum):

Planted alongside tomatoes, both plants promote each other’s growth, and the essential oils of the basil repel aphids and whitefly and help prevent powdery mildew.

Parsley (Petroselinum crispum):

Keeps various types of aphids away.

Note from Thorsten: It also grows brilliantly right through winter. Just recently I harvested parsley in December for falafel and chimichurri.

Borage (Borago officinalis):

Acts through its scent against cabbage white butterflies, the garden dart moth and the tomato hornworm. Does not get along with parsley.

Thyme (Thymus vulgaris):

Also works against cabbage white butterflies, whitefly and aphids. Antiviral and antibacterial properties are also attributed to it, and it is suitable as a border planting both outdoors and in the greenhouse. There it also deters slugs. Some say it repels slugs; in our garden it tends to attract them so you can collect them there. Perhaps we have gourmet slugs 😉.

The list of helpful plants can certainly be extended further…

Green Manure with Phacelia (Phacelia tanacetifolia) and Others

Once you have properly cleared out the greenhouse, a green manure is recommended. Phacelia is a good choice, as it is not related to any typical vegetable plant. For nitrogen input, legumes such as vetches, peas and lupins are suitable. Crucifers such as mustard varieties or oilseed radish contribute to a healthy soil with their mustard oils. Good mixtures, including with other plants such as buckwheat, can be found from seed merchants.

In spring, cut or mow the green manure plants and work them lightly into the soil as a surface compost. Chopping them finely allows the soil organisms to process them more quickly. You can also use the plants for mulching, which keeps the soil covered. This brings additional benefits – moisture stays in the soil longer and unwanted weeds struggle to establish. There is a little more on mulching on the page about row companion planting by Gertrud Franck.

This sustainably improves the soil. Nutrients are bound and made bio-available. And the tight crop rotation is broken up.

Growing Everything in Pots

Alternatively, a greenhouse can be used differently by planting all or some of your crops in pots, automatically giving them partly fresh substrate each year. For this, the pot size must match the needs of each plant. Err on the side of larger rather than smaller. For smaller pepper plants, at least 10 litres; larger pepper plants and dwarf tomatoes need at least 15 litres, and anything above that needs pots of at least 20 litres.

Of course, the space and therefore the nutrient supply in a pot is less, and you will need to fertilise more often. Pots also dry out much more quickly if you are not careful and water regularly. This can cause drought stress in the plants, leading to flowers and fruits dropping off.

Depending on which substrate you fill the pots with, it can also become costly to refill them every year.

Summary

I think the way you use a greenhouse is a matter of personal preference. We do a mix of both. Some plants go directly into the ground, others are grown in pots. Smaller secondary crops in particular can still be grown on shelving or the potting table. We also start successive crops for outdoors and for the greenhouse there.

We try to follow the methods described above, which are advisable for an organically run garden anyway.

If yields in the greenhouse drop after a few years despite these measures, or if diseases do take hold, you will probably have no choice but to replace at least part of the soil. But you can probably put off that laborious job for a while through good companion planting and targeted soil improvement measures.