How to Water Grow Bags: Bottom Watering or Drip?
Bottom watering grow bags or running drip? Put the big bags on drip and keep the bottom soak for small bags of herbs and greens. Here's what researchers found when they pulled pots apart layer by layer, and how to pick for your own bags.
We didn't run this test ourselves; greenhouse researchers already did. In green beans grown in pots about nine and a half inches tall, drip spread the water evenly, about 26% by weight from top to bottom, while bottom watering held 29% in the bottom layer and only about 16% in the middle and top. The bottom-watered pots ended up with a top layer about ten times saltier, most of their roots sat in the bottom and middle, and the drip-watered beans carried about 52% more root weight. Harvests mostly went to drip: about a third less from the bottom-watered beans, 9.5% less in a Mexican tomato trial, and a smaller, higher-quality crop of Italian cherry tomatoes. Bottom-watered tomatoes came out ahead in one trial, but only once the fertilizer was cut to about 60% strength, so if you bottom water, stay at the low end of the label rate and mix slow-release fertilizer through the bag. The water savings belong to greenhouse systems that pump the leftover back. A tray under a grow bag doesn't recycle anything, and we couldn't find a single test of a fabric grow bag sitting in a tray. Bottom watering still earns its keep in three spots: a drier surface is a nudge against fungus gnats, a short bag of herbs or greens is an easy climb for the water, and a bag that's dried out hard comes back with top watering a little at a time or a soak halfway up in a tub, not an inch of water in a tray. For the big bags, give each one two or three emitters, run each cycle until a little water drains out, top the mix with an inch or two of mulch, and put a backflow preventer first in line on house water. With toddlers or pets around, skip standing water altogether: mosquitoes can go from egg to adult in about seven to ten days, and buckets of water are a drowning hazard for small children. Sources: Bouchaaba et al. 2015 (Scientia Horticulturae), Pennisi & van Iersel 2000 (University of Georgia), Santamaria et al. 2003 (Journal of Horticultural Science & Biotechnology), Méndez-Cifuentes et al. 2020 and García-Santiago et al. 2019 (Water), Goodwin et al. 2003 (Australian Journal of Experimental Agriculture), Schulker et al. 2020 (Agronomy), Spomer 1976 (Container Soils Are Different), Virginia Cooperative Extension, Colorado State University, University of Maryland Extension, the CDC and the U.S. Consumer Product Safety Commission.
0:00 Tray or drip for grow bags? 0:40 How does each one move water? 1:26 How far up does the water get? 1:57 Where does the salt end up? 2:54 Does bottom watering grow deeper roots? 3:28 Which one grows more food? 4:29 Does a tray save water? 5:13 When is bottom watering the right call? 6:39 Why is drip the system for big bags? 7:19 So which one should you pick?
More on watering grow bags: Grow Bags Drying Out? The Cheap Drip Timer Fix — https://www.youtube.com/watch?v=iO9f2oy2EQE DIY Self-Watering Grow Bags: 3 Cheap Hacks Compared — https://www.youtube.com/watch?v=hSISHB8DI2A How to Get Rid of Fungus Gnats Naturally (and What Doesn't Work) — https://www.youtube.com/watch?v=CJLzZq2VtxU
Drip, trays, or a watering can and good intentions? Tell us what runs your patio in the comments, and subscribe for more garden arguments settled by the people who actually measured.
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