Depth, drainage, and not over-reading the guides
Last lesson was all about volume math, filling the bed, not buying more soil than you need. This lesson is about what happens after the dirt's in there. Depth and drainage. Two things people either ignore completely or way overthink, and I've been on both sides of that.
How deep is deep enough
Here's what actually happens with root depth. Most of the vegetables you're growing for a fall or winter harvest, spinach, lettuce, carrots, beets, don't need nearly the soil depth people assume they do.
Rough numbers I use:
- Lettuce and spinach — 6 to 8 inches of good soil is plenty. These are shallow feeders.
- Carrots and beets — 10 to 12 inches, and honestly that's generous. Carrots will tell you if they're cramped, they'll fork and twist, but they don't need two feet of soil to do it.
- Anything in a cold frame you're planning to keep going into November — I don't go past 12 inches unless the bed was already that deep for other reasons.
I mention this because I built a bed a foot too deep once, over-read the guides, didn't stop to think about what that meant in wheelbarrow loads. Eleven trips and a second Macey's run for bags. The bed's fine. I just filled a lot of space with dirt that no root in that bed was ever going to reach.
So before you dig deeper, ask what's actually going in the ground. Root crops earn a little more depth. Greens don't. Run the numbers on that before you commit to a depth, not after.
Drainage matters more than depth
If I had to pick one of the two to get right, it's drainage. A bed that's shallow but drains well will outperform a deep bed that stays soggy, every time.
Signs your drainage is bad:
- Water sits on the surface more than a few seconds after you irrigate.
- Soil feels heavy and cold to the touch a day after watering.
- Plants look wilted even though the soil's wet, which is the opposite of what people expect and it throws beginners every time.
For raised beds specifically, the fix is usually simple. Make sure the bottom of the bed isn't sitting on hardpan or compacted clay, which is common out here once you get a few inches down. If you built on top of existing ground, take a shovel and loosen the native soil under the bed before you fill it, even just six inches down. Otherwise your raised bed is basically a bathtub.
I'll say this again because it's one of my actual opinions and I stand by it: hard water and dry air are the two things nobody warns you about at this elevation. The hard water clogs drip emitters over a season (took me a whole summer and a vinegar soak to figure that one out), and the dry air pulls moisture out of your beds faster than the seed packet math accounts for. Drainage isn't just about getting water out. It's about managing water you're going to have to add back more often than you think.
Don't over-read the guides
This is the part I want to slow down on, because I see people trip on it constantly, myself included.
The extension office charts, the seed packets, the gardening books, they give you numbers that are correct in general and wrong for your specific bed. A guide will tell you carrots need 12 inches of loose soil. Fine. But it won't tell you that your particular bed drains slow because it's sitting on old construction fill, or that your soil mix is heavier than the bag suggested.
Honestly, I still catch myself over-checking things I should just trust by now, soil temp especially. I'll tell you the same thing I'd tell a beginner: read the guide for the range, then watch your actual bed for the answer. If your lettuce is doing fine in 6 inches of soil that drains well, you don't need to dig it deeper because a chart said 8. If your carrots are struggling in 12 inches because water's pooling underneath, more depth isn't the fix, drainage is.
The guides are a starting point. Your yard has the final word.
A quiet habit that's paid off
I take a photo of the harvest most years, right on the back step in morning light. Nothing fancy, I'm not trying to make it look like anything. I just like having a record of what a good bed actually produced, because come February when everything's empty and gray, it's easy to second-guess whether last year's setup was actually working. Having the photo settles the argument. That bed with the drainage I fixed halfway through the season, the one I almost gave up on? The picture reminds me it came through fine once the water started moving.
It's a small thing. But if you're experimenting with depth and drainage this year, take a picture of what comes out of each bed. You'll want the evidence later, whether it's good news or a lesson.
Before next time
Go check your existing beds after the next time you water. See how long standing water actually sits on the surface, and note it somewhere. That's your real drainage number, better than anything a guide will give you.