So How Did we Design the Great GROW Experiment?

The three sisters for Europe?

Dr Naomi van der Velden, Permaculture Association (Britain)

Finding a three sisters for Europe — a polyculture that grows well in this part of the world — was no easy task. Here Naomi van der Velden explains the thinking behind the choice of spinach, green beans and radish.

Polycultures — growing several crops together at the same time and in the same place — can offer lots of benefits, as we discussed in our first Experiment blog.

However, with the exception of the notable “three sisters” of maize, beans and squash in Latin America, there has been relatively little scientific research into combinations of three or more crops. Through GROW, we have discovered too that growers have a great interest in polycultures and companion planting, but, compared to other regenerative growing practices, the least certainty about how well they work.

The GROW Experiment has therefore been designed to compare three crops grown in a polyculture with those same three crops grown separately in monocultures. We’ll examine how productive they each are both from an individual grower’s perspective and as a combination of all of our plots. We will consider what factors might account for any differences we see. Because an earlier study suggested that sweet corn (maize) doesn’t grow fast enough or well enough for beans to climb in more temperate regions, the classic three sisters was not chosen. Instead, we wanted to select a mix that might be more suitable to these conditions.

We’ve been getting lots of questions from our fabulous experimenters about the choice of crops and the planting layout. We’re very pleased to be able to answer these more fully here. If you have further questions, please pop them in the comments below or email us on or join the GROW Experiment discussion forum:

Choosing the crops — selection criteria

In the early design stage, we invited growers and experts to suggest some combinations, then further refined suggestions according to these logistical needs and plant ecology traits:


  • Grows in many countries
  • Can be planted and harvested May — October
  • Can grow several plants in a relatively small space (1 metre-squared or less)
  • Short growing season for flexibility in when people join
  • People like to eat it!


  • Similar habitat requirements (soils, climate) so can grow in same conditions
  • Have complementary spatial niches to avoid negative interactions like competition, and make best use of ground space
  • ‘Companion plant’ benefits likely — positive interactions between the three.
Planting and harvesting times for French beans, spinach, and radish across 9 European environmental zones

We examined planting and harvesting timings for crops across nine key European environmental zones, looking for crops that best fit our May to October time slot.

Because of the variation in zones, especially in the very south (MDN — Mediterranean north) and west (LUS — Lusitenean) it was not possible to find crops that suited every environment perfectly.

Three edible species were chosen based on these planting and harvesting timings, complementary spatial niche occupancy, ecological requirements and their food quality (or how much we like to eat them!).

You can see the expected planting and harvesting timings for these in the planting calendar image (left).

If these calendars don’t match what you normally do, see how you can help improve them at the end of the blog!

The crops selected

Climbing green beans (french beans), spinach and radish were selected. Climbing beans make good use of vertical space, are in the top 10 “most delicious vegetables” and are nitrogen fixers, meaning that if you leave the leaves and stems to die back after harvesting the beans, they can enrich your soil with nitrogen. They like reasonably warm temperatures to grow and good light levels to produce well. They prefer fertile well-drained soils that are loamy to sandy and neutral to alkaline pH. They grow best in south and west aspects in a sheltered site, after danger of frost has passed.

Spinach grows in clumps and gives good ground cover. The leaves can be cut regularly as baby leaves over a long time, and are tasty and nutritious. It’s also in the top ten of favourite vegetables. It grows best in rich fertile soils and can be sown in both spring and autumn. It also enjoys well-drained soils, and does well on loams and clays that are neutral (pH 7) to alkaline (higher pH). Growing in partial shade can help prevent it ‘bolting’ (growing rapidly into a flower), especially in dry weather. It’s therefore ideal to plant between rows of a taller crop like beans. They grow best in south and west aspects in a sheltered site and are tender in frost.

Radish is a root crop, although if left to flower it produces a large number of fleshy seed pods which are also edible and have a milder flavour than the root. It can be sown in succession (e.g. every 2–3 weeks) to spread out the crop and avoid a large glut. They like good amounts of sun, so are best to grow around the south and west edges of the plot. They prefer moist well-drained soils (loams to clay) with neutral to alkaline pH. They grow best in south and west aspects in a sheltered site, and are somewhat prone to frost damage.

The particular varieties of “cobra”, “matador” and “cherry belle” chosen were selected as they have good all-round performance. Region specific searches by country determined that they are widely available to buy as seeds across, and beyond, Europe at a reasonably low cost. They are also available as organically-certified seed. It was noted that these varieties were not available for every country, so flexibility for people to select similar varieties that are more accessible in their area was incorporated into the experiment design.

Determining the planting arrangement

Spacing requirements were determined based on general planting guidelines — space between plants, and space between rows. Monocultures were spaced according to these general planting guidelines, with average spacings used when guidance from different sources varied. SPinach was spaced slightly closer in the monoculture than the recommended guidance since it was intended as baby-leaf or “cut and come again” rather to be than for whole plants to be harvested or for the plant to reach maturity (flowering).

These usual planting guidelines assume that you will plant a block of several rows of a crop (a monoculture), so these densities have to be modified for the polyculture planting. The likely above-ground growth of the crops (in height and spread) were considered, along with consideration of each crops tolerance to share (or preference for high light). One of the advantages of polycultures should be that more crops can be grown in the same space, so accounting for how much shade the taller crops create and the extent to which shorter crops tolerate shade (spinach can benefit from light shade, radish prefers full sun), is important.

Typical root development, based on Bruner (1927), was also accounted for as this can affect competition for water and nutrients.

Space and light needs for each crop and suggested polyculture minimum spacing:


  • space between plants 20 cm
  • space between rows 40 cm
  • roots depth to 140 cm, greatest density 15–60 cm
  • prefers full sun
  • polyculture spacing ~30 cm, main canopy layer


  • space between plants 5 cm
  • space between rows 30 cm
  • root depth to ~60 cm, greatest density 0–25 cm
  • can grow well in partial shade
  • polyculture spacing ~10 cm, sub canopy layer, below beans
  • monoculture spacing — 10 cm, reflecting cut-and-come-again nature of harvest (not removing entire plants).


  • space between plants 2.5 cm
  • space between rows 15 cm
  • roots deep (to 80 cm), greatest density 8- 30 cm
  • prefers full sun
  • polyculture spacing — 5 cm, root layer, around edge

Density — number of seeds per area

Perhaps the most challenging aspect to calculate is the relative densities and the optimum number of seeds of each crop. In usual intercropping trials of two crops, space is normally set in advance and then the seeds can either be planted in one of two patterns; substitution or addition.

A “substitutive” design mimics the density of one crop as grown in monoculture e.g. a row of crop B is planted at the same density as a row of crop A. Here the density (number of plants per square meter or ha) remains the same in the polyculture and one monoculture, and the ratio of one crop to the other differs (when compared to accompanying monoculture plantings).

Substitution of crop B into A to match monoculture spacing of crop A

Alternatively, an “additive” design can be taken where the two crops are planted at their usual monoculture density but added together. Here the density of the polyculture is always higher than that of the least dense monoculture.

This becomes complicated when more crops are included. We opted to maintain the total number of plants (since the number of plants grown can influence total yields), and to try to keep the densities in line with the plant spacing requirements. To achieve this, we altered the total area being grown in so this differs between polyculture and monoculture. It will be relatively easy to later calculate yields per square metre (or per ha).

Planting area

The area needed to grow in is a balance between the experimental requirements — having enough replication of plants to get good results — and the practical requirements — not asking growers for too much of their precious space. We worked to an area of approx. 2 square metres as a balance between these requirements.

Starting with a 1 metre square area for the polyculture, we calculated the optimal numbers of each crop to align with the spacing requirements and allow for healthy plant grow, whilst appreciating the value of overlap in the case of the spinach.

Trying out densities and calculating the number of seeds of each crop needed.

From this we had the ideal number of seeds — 9 bean plants, 36 spinach and 80 radish. The monoculture spacings were then calculated to accommodate these, using the 1 metre length and adjusting the width for each crop:

  • to 60 cm to allow adequate spacing of 30 cm between the bean plants;
  • to 40 cm for the spinach, and
  • to 30 cm for the radish)

giving a total area of 1.3 square metres for all the monocultures.

There was one final consideration — the effect of the plots and any other crops or vegetation nearby on growth. Ideally, the spacing between the plots would be equal to the maximum height of the vegetation ie. 2 metres as this is how high the beans can grow. We determined that a minimum spacing of 50 cm would strike a balance between the scientific ideal and the practical needs of growers.

Hence, the final planting arrangements for the polyculture and monoculture plots as shown below:

Planting arrangements for the polyculture and monocultures in the Great GROW Experiment (though best to plant the beans to the north of the spinach so they don’t shade it!)

Can you help update the planting calendar?

Since we’ve started the experiment, there’s been lots of feedback from people about their usual planting times that don’t seem to support the available information — as shown in the calendar image above. Time for citizen-science to come to the rescue again!

GROW is working on improving the “official” planting times based on on the ground experience from our many participants. Do you start planting or harvesting earlier than these times? If so, please do share your knowledge to help other local growers make better choices — and to improve our experiment for next year!

Good news! We now have this incredible resource, our “Share My Planting Calendars” facebook group!

Join the facebook group and then click on the image of the vegetable or crop you’d like to share info for and add in:

Place: e.g. Carlisle, UK

How I grow: e.g. sow indoors ~15 April — ~20th May

When I sow outdoors: e.g. ~26th May to ~15th June (later when still frost-risk)

When I harvest: e.g. ~15 July — September.

We’d love to hear your suggestions and questions — you are more than welcome to join the GROW Experiment discussion forum, or email us on