How to Water Grow Bags: Bottom Watering or Drip?

That tray of water under your grow bag might be quietly piling salt into the top of the mix, about 10 times more than drip does, if the researchers who tested both are any guide. Bottom watering versus drip is one of those garden arguments that never ends. One side swears the tray grows deeper roots, chases off fungus gnats, and saves water. The other side just wants a timer and a nap. We didn't run this test ourselves. Researchers already did, in greenhouses, pulling pots apart layer by layer. And what they found is more useful than a winner. It tells you which method fits which bag. If fewer droopy afternoons sounds like a good deal, hit subscribe. First, how each one moves water. Drip puts it on top, a slow trickle that gravity pulls down through the bag. Run it until a little drains out the bottom and that drainage carries extra fertilizer salt out with it. Bottom watering works the other way around. The bag sits in water and the mix pulls it upward through tiny pores, the way a paper towel drinks a spill. That's capillary action, and the higher the water has to climb, the less of it arrives. Every container also keeps a soggy layer at the very bottom, even with good drainage holes. Soil scientists call it a perched water table. And as one classic nursery science article put it, the deeper the container, the drier its surface after watering. A grow bag is a deep container. So how far up does the water actually get? A greenhouse team in southern Italy grew green beans in pots about nine and a half inches tall, some on drip and some in a trough that flooded from below. Then they took the mix apart in layers. On drip, the water was spread evenly, about 26% by weight from top to bottom. Bottom watered, the bottom layer held 29% and the middle and top only about 16%. And that's in a pot shorter than the 10 and 15 gallon bags most of us put tomatoes in. Now the salt. To a gardener, that means the dissolved minerals that fertilizer and tap water leave behind. When water climbs up from below and evaporates off the surface, the fertilizer salts it was carrying stay behind on top. That's how the bottom watered These watered beans ended up with a top layer about 10 times saltier than the drip pots. University of Georgia researchers saw the same pattern in potted poinsettias, with salt in the top layer up to 10 times higher than the layers below. And roots don't care for a salty neighborhood. In bottom watered pots, most of the root growth sat in the bottom and middle. You can rinse it out from the top, but it takes a real soaking. The Georgia team found that a small splash just washed the salt down to where the roots were. To actually move it out the bottom, they had to pour through more water than the pot held mix. For a 10 gallon bag, that's a lot of watering can. Drip does this chore for free, every time a little runs out the bottom. Which brings us to the big one. Here's the garden lore. Bottom watering trains roots to grow deeper and stronger. Here's what the research says. Roots go where the water is. In a bottom watered pot, they crowd into the bottom and middle, and the dry, salty top gets the fewest. As for stronger, the drip watered beans in that Italian study carried about 52% more root weight than the bottom watered ones. Nobody's training anything. The roots are just following the water. And in a grow bag, deeper only goes as far as the bottom seam. Does any of this show up at harvest? In the bean study, the bottom watered plants yielded about a third less. A tomato trial in Mexico, in plastic containers of about three and a half gallons, put bottom watering nine and a half percent behind drip. An Italian cherry tomato trial found a smaller crop too, though the fruit scored higher on quality. And here's the twist. Another trial from that same Mexican research group put bottom water tomatoes ahead of drip, but only once the fertilizer was cut below full strength. Their recommendation was about 60% strength. So if you bottom water, don't feed heavier to make up for it. Stay at the low end of the label rate. And mix slow-release fertilizer through the bag instead of sprinkling it on top. In an Australian trial, when the fertilizer sat on the surface, plants on capillary mats grew smaller than plants watered from above. Mixed through the potting mix, tomatoes and roses on the mats grew as big as anything on drip. What about saving water? This is the argument bottom watering is supposed to win. And in the greenhouse, it does. With one catch. Those systems catch the leftover water and pump it back for the next round. The Italian cherry tomatoes used about 5 gallons of fertilizer water per pound of fruit that way, against about 7 on drip. A tray under your grow bag doesn't recycle anything. Without that trick, the savings mostly disappear. In that Australian trial, plants on capillary mats, the closest thing to a tray, used about the same water as drip. A touch more, actually. And we couldn't find a single test of a fabric grow bag sitting in a tray, so that part's still an open question. So is bottom watering useless? Not at all. It's a good tool. It just isn't a good system for a big bag, and it earns its keep in three spots. First, fungus gnats. Their larvae hang out in the top two or three inches of damp mix, and Colorado State says letting the surface dry kills the eggs and larvae. Bottom watering keeps that surface drier. Nobody has counted gnats under drip versus bottom watering though, so call it a nudge, not a cure. Second, small bags of herbs and greens. A short bag is an easy climb for water and it's light enough to carry to a tub. Virginia Tech's extension advice for bottom watering potted plants is two to three inches of water for 15 to 30 minutes. Never more than an hour or root rot could set in. Then lift it out and let it drain. And the one people get backwards is the rescue. A bag that's dried out hard looks like the perfect thing to stand in a tray. But in a North Carolina state lab test, dried out peat watered from below 10 times in a row only got back to about 16 percent moisture. Watered from the top the same 10 times, it reached about 38. Dry peat just doesn't wick well. So use the rescue Maryland's Extension Service gives. Water from the top a little at a time until the mix starts drinking again. Or sink the bag halfway up in a tub of water, which is a very different thing from an inch of water in a tray. For most grow bag gardens though, drip is the system, especially for the big stuff. Tomatoes, peppers, and eggplant in 10 or 15 gallon bags? A whole row of them. Drip wets the bag evenly, it carries the salt out every time it drains a little, and one timer runs the lot. It isn't set and forget though. Clean the filter now and then, and nudge the run time as the weather changes. Give a big bag two or three emitters around the plant, not one. Run each cycle until a little water comes out the bottom, and top the bag with an inch or two of mulch so the sun isn't baking the surface. And if it's hooked up to your house water, a backflow preventer goes first in line. So, here's how to pick. Tomatoes or peppers in big bags? More than a handful of bags? Or gone all day? Drip on a timer. A few small bags of herbs and greens and you're around most days? A bottom soak works fine, then let it drain. Hard water or lots of liquid feeding? Drip, because it carries the salt out. Want a real reservoir? That's a self-watering container with an overflow hole, which is a different build from a bag and a tray. And if there are toddlers or pets around, skip the standing water altogether. The CDC says mosquitoes can go from egg to adult in about seven to ten days, and the Consumer Product Safety Commission warns that buckets of water are a drowning hazard for small children. Dump trays and tubs when you're done. What runs your patio? Drip, trays, or a watering can and good intentions? Put your setup in the comments. And this time of year, across the South, the Southwest, and the West Coast, Fall greens and herbs are growing in small bags, exactly where a bottom soak works best. Cooler days mean slower drying, so cut the timer back and check with a finger before you water. Everywhere else, before your first hard freeze, drain the drip line and bring the timer and the filter inside. Let the drip line carry the big bags, keep a tub handy for the herbs and the And let that top inch of mix dry out, the way the gnats hate.