Question
Hi Will,

Previous experience companion planting different species of plants in the same pot has been met with varied success, the most successful being plants with similar light and water requirements.

Recently, I read on a garden forum that plants of widely differing cultivation requirements can do well planted together if they are of different sizes, specifically the one needing more water being the larger. The explanation offered was that the roots of the larger plant "will take out the extra moisture", presumably causing the soil to dry before root rot affects the smaller plant.

As rational as this seems on the surface, intuitively it is incorrect (and contrary to personal experience), yet I am at a loss to scientifically explain why.  Can you shed some expert light on the matter?

Regards,
Aileana


Answer
Hi Aileana,

In my experience, online garden forums are fraught with misinformation by well-intentioned people who offer up advice based on personal, anecdotal experiences and not based on professional or scientific knowledge. I once spent time on some of the popular garden forums, but found that my professional experience was often given no greater credibility than that of someone with limited plant experience.

Plants do not compete with one another for space in a pot or for water or nutrients. Nor do larger plants absorb more water and keep smaller plant roots drier.

When you water a plant properly, the entire rootball is saturated equally. All roots in that rootball have the same exposure to moisture. As time passes, some of that water is absorbed by plant roots and some of it evaporates. But the moisture level throughout the rootball is relatively constant. Although the top of the rootball dries out sooner due to evaporation, the moisture content around all roots is consistent. It is not drier near small plant roots and wetter near large plant roots. That makes no more sense than suggesting that high light plants will absorb more of the sunlight and thereby protect the low light plants nearby.

I am amazed at the stuff people will make up and then report as fact!

You are correct that companion plantings work best when the plants have similar light and water requirements. Plants in the same pot will have the same light and water exposure. That means you cannot successfully keep a high-light-low-water cactus together with a low-light-high-water peace lily.

Please let me know if any of this is unclear or if you have any additional questions.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

You can E-mail me directly at: [email protected]

P.S. You have a very interesting name!