The soil in a raised bed has open air along its sides, not just across the top. Ground-level soil is packed in by the earth around it and a wooden frame is not, so the whole box heats up faster and gives up water faster. That is the short answer for a raised bed drying out while the row garden a few steps from your frame stays damp. Heat is the cause here, not thirsty plants.
The zucchini leaves lay flat on the soil by mid afternoon, and those same plants had stood up fine all spring. I checked it with a finger and the top few inches were dust, while the layer below my knuckle was still damp. So I pulled the hose off the bib at the corner of the house and watered in the middle of the day, for the third time that week. Hot soil sheds water much faster than warm soil, and Utah State University Extension puts more frequent summer watering right beside that fact.
Warm soil hands water to the air faster than cool soil does. That process is evaporative moisture loss, and a bed with open sides collects more of it than flat ground does. Utah State scopes that loss to the hot summer months, when the soil runs warmest. Raised beds also drain well, which is one of the reasons people build them, so water leaves through the bottom while the sun takes it off the top.
The missing insulation is not a flaw in your build. It is the same thing that lets a bed warm up early in spring, which buys you an earlier planting date and a longer season than open ground can offer. You cannot keep the head start in spring and skip the extra watering in the heat, because both come out of the same open sides. That makes this the price of a real benefit, and I think it is a fair trade if you like getting seed in the ground early.
Good raised bed irrigation is less about how much water you pour and more about how the water gets there. A raised bed garden has to be irrigated, and drip or trickle line is the most efficient method you can put in. Cover the soil as well. Mulch and cover crops cut evaporation and help water soak in instead of skating across a dry surface.
- Irrigate: Plan to water the bed yourself. A raised bed garden has to be irrigated, and rain alone will not carry it through a hot stretch.
- Drip line: Run drip or trickle line down the rows, which puts water at the roots and wastes little of it to the air.
- Mulch: Cover the bare ground between plants. Mulch cuts evaporation and helps water soak into the soil.
- Cover crop: Sow a cover crop in the gaps between plantings so the bed is never left with open soil.
- Raise the watering frequency in hot summer weather and lower it again once temperatures drop.
Get both pieces in place before the hot stretch arrives, not after the first wilt. The line carries the water to your plants, and a cover of mulch keeps it from leaving, and neither one does the whole job alone. Bare soil also swings in temperature and erodes, so leaving the ground open costs you twice.
Set your drip line up in spring, while you are still enjoying the early warm soil. The same open sides that handed you that head start are what will drain the bed in the heat.
Water your bed more often while the heat lasts, and ease off once it breaks. The bed is not broken. It is doing what an uninsulated box of soil in full sun does, and it hands you an early spring in return.
Read the full article: Raised Garden Bed Specs Backed by Research
