Update: Polyculture Groups Appear Healthier Than Monoculture Counterparts

(Zinnia elegans foliage with blight from monoculture group)

After working with severally very different species of plants, including vegetables, fruit, flowers, and other varieties, I have definitely observed some trends. In areas where one species was grouped together en mass (monoculture), the plants have been suffering from the rapid spread of disease and have suffered high numbers of insects consuming the foliage. However, plants that are more isolated from their own kind (such as ones with different species of plants in between) have fared much better. This was expected, since the physical movement of pests and diseases is made easier by proximity, and I saw this occurring with several species. The Zinnias (Zinnia elegans) that were planted in large groups have been showing signs of blight on the majority of the individual plants. However, Zinnias that are more isolated, with other plant species in between them, did not develop the blight.

Here is a photo of one of the isolated zinnias, which exhibits very healthy foliage and has no apparent signs of disease:

(Healthy isolated Zinnia plant)

On the other hand, the Zinnias from the group that were grown together showed significant signs of disease:

(Zinnias from the monoculture group suffering from Alterneria blight)

Several Zucchini (Cucurbia pepo cv zucchini) were planted in a large group, while other zucchini (also Cucurbia pepo cv zucchini) were separated by rows of French Marigold (Tagetes patula) and Marigold (Calendula officinalis). The Zucchini planted together en mass were consumed, entirely, by cucumber moth caterpillars (Diaphania indica).

(Diaphania indica larvae)

The Zucchini that were separated by rows of marigold fared slightly better -- they also had several caterpillars, but not enough to completely destroy the plant as the other group had. Interestingly enough, the zucchini plants that were surrounded by the most marigold foliage had the fewest caterpillars present.

(Adult Cucumber Moth - Diaphania indica)

Even in plant species such as Milkweed (including Asclepias curassavica, Asclepias incarnata, and Asclepias tuberosa) exhibited a sharp decline when grown in a monoculture environment, interestingly. Milkweed is considered a weed, and to many farmers, a pest. However, it is the larval food source for the Monarch butterfly, which has been in alarming decline, and its relative the Queen butterfly. When milkweed plants were grouped together, large numbers of aphids and milkweed bugs (Lygaeus kalmii) were found present, and many of the plants either perished entirely or were very stunted in their growth.

 

(Milkweed bugs and aphids on the monoculture group of A. curassavica)

Other milkweed that were located away from the main group, among different species of plants, were covered in far less aphids and rarely had milkweed beetles on them. The isolated plants were much healthier and grew much more quickly. Although Monarch larvae were present on both and consumed a portion of the leaves of either the isolated milkweeds or the group of milkweeds, the isolated milkweeds had a much faster recovery time for putting out new shoots and leaves. Individual milkweed plants located near lavender (Lavandula multifida) had almost no aphids, and there were no beetles present.
   

(A healthy isolated Tropical Milkweed plant, A. curassavica)

The only group that did not exhibit this trend was the potato and sweet potato groups and the hibiscus group (Hibiscus sabdariffa). No evidence of pest or disease was found in any of those groups, possibly because soil drainage was very good and because these plants are less disease- and pest-prone than many other crops. The sunflowers (Helianthus spp.) were overall healthy when planted in a group, and did not suffer any disease, but small portions of the leaves were consumed by moth larvae. Isolated sunflowers did not appear to have any portion of their leaves consumed.
   

(Healthy sweet potato plant - Ipomoea batatas - from monoculture group)