Ill.: Park Seed
I receive a lot of email from readers putting in raised bed gardens, but who seem doubtful that you really need to fill the whole thing from the base to the summit with top-quality soil. They don’t seem to realize just how deep vegetable roots can go. I have beds 2 feet (60 cm) high filled with the best soil I could buy locally and I find that scarcely enough, at least, not when you want productive vegetables rather than a wimpy crop.
Some people even want to put a layer of gravel in the bottom. Gravel! Can you believe that? Or fill it with so-called “black earth” (actually black peat): inexpensive, for sure, but just about the worst soil amendment on the market. Yikes!
I think it might help to see just how deep some vegetable roots grow, as the chart below shows. Maybe you can skimp when it comes to herbs (most are shallow-rooted and not particularly needy of rich soil), but when it comes to vegetable gardening, you always want the best soil you can get and as deep as possible.
Soil Depth Requirements for Common Garden Vegetables
| Shallow Rooting 12?–18? (30–45 cm) | Medium Rooting 18?–24? (45–60 cm) | Deep Rooting 24?–36?+ (60–90 cm+) |
| Arugula | Bean, dry | Artichoke |
| Basil | Bean, fava | Asparagus |
| Bok Choy | Bean, pole | Bean, lima |
| Broccoli | Bean, snap | Comfrey |
| Brussels sprouts | Beet | Horseradish |
| Cabbage | Carrot | Okra |
| Cauliflower | Chard | Parsnip |
| Celery | Cucumber | Pumpkin |
| Chinese cabbage | Eggplant (aubergine) | Rhubarb |
| Chives | Kale | Squash, winter |
| Cilantro (coriander) | Melon | Sweet potato |
| Collards | Pea | Tomato |
| Corn | Pepper | Watermelon |
| Endive | Rosemary | |
| Fennel | Rutabaga | |
| Garlic | Squash, summer | |
| Jerusalem artichoke | Turnip | |
| Kohlrabi | ||
| Leek | ||
| Lettuce | ||
| Mache | ||
| Mint | ||
| Onion | ||
| Oregano | ||
| Parsley | ||
| Potato | ||
| Sage | ||
| Savory | ||
| Spinach | ||
| Strawberry | ||
| Tarragon | ||
| Thyme |
You still want to fill your raised bed with third-quality soil? Or black earth? Or gravel? Go ahead, but don’t say I didn’t warn you!
Do not agree with any of these depth numbers. If they were true, then no raised beds would be practical or functional. Example: 24-36 inches deep for pumpkins! I grew pumpkins in a field and then pulled them up in the fall. Roots were not even a foot deep.
Do you have some sort of illustration for vegetable root depths that you believe are accurate? or just could type them out and post? or send? Would be so helpful. Abby
Or just put it in the ground.
I hear the same about gravel. Some believe it is good for potted plants. I suppose it can be useful in pots if there is something on top of it to prevent the airspace from filling with medium. A landscaper told me that he puts large stones and bricks into large pots to make them too heavy to steal (as if the stones and bricks are significantly heavier than wet medium). I think that if I were concerned about pots being stolen, I would not use pots.