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Root Pruning: Is it worth the time?

Roots are the most important yet often overlooked structural component of a healthy plant. In comparison to mammals, they are the legs, feet, mouth, eyes, hands, and in a way the brain. They provide stability, search out nutrients, provide water, send hormones and signals to the rest of the plant, and can even defend against other plants invading their space. We often look at pruning as something we do to maintain the visible parts of the plant; trimming “suckers” below axillary buds on tomato plants, thinning crop canopies to allow light for fruit ripening, or removing flowers to encourage stronger root growth. Often overlooked is the practice of root pruning.

The idea behind root pruning is to trim any “overgrown” or circling roots before transplanting so that the roots grow out in the right direction and increase the surface area of roots that can uptake nutrients. The real science behind this theory is that roots perform different functions as they grow and age. Older, thicker roots primarily act as structure for the weight of the plants and to transport nutrients from far-reaching smaller roots to the stem. Younger, more fibrous roots are primarily performing the function of bringing in water and nutrients. In a way roots become similar in design to highways, avenues, and side streets; the streets being fibrous roots that converge on avenues and highways (taproots). There is a minimum number of “structural” roots required to maintain plant structure, but in hydroponics most crops do not require as many because there is so much less environmental stress. This makes it logical to want to encourage as many “fibrous” roots as possible to increase the amount of nutrient uptake and subsequent yield.

When older roots are pruned, the hormones that signal young root growth start to accumulate at the cut site instead of being distributed to young root tips. Once the wound calluses and heals, new roots will begin to form at the wound site. These small roots are “adventitious” and fibrous, so their primary function is finding sources of nutrients and water, not structural support. If root pruning is done at the correct level and timing, a yield increase in some crops can be observed at levels that are worth exploring in your farm. In a recent experiment conducted in our research farm, we tried root pruning circled tip roots (middle photo below), half the rooted plug (right photo below) and compared their growth habits and yield to a control with no pruning (left photo below) on Rouxai Lettuce.

Once harvested, we calculated the average weights of the 2 treatments and the control to determine the difference. We found simply trimming excess roots from the bottom of the plug that have circled in the tray gave us an average weight increase of nearly 8 grams or 8%. That may not seem like a significant amount, but if you factor in that a full ZipFarm™ can grow on average over 500 kg of lettuce in 3 weeks, that’s an extra 40 kg of lettuce every harvest (assuming the whole farm was planted at the same time). In total that works out to over 600 kg of extra lettuce per year!

This may not work with all crops but is worth experimenting on your varieties to see if you can improve yields and crop health. Root structure and health is often an overlooked pillar of farmer success, and by encouraging healthy root growth you can see major improvements to your farm yields.

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