Herbs Vegetables

Root Depth for Vegetables

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.

In a raised bed, you want rich, well-draining soil from bottom to top. Photo: The Gardening Channel With James Prigioni

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+)
ArugulaBean, dry Artichoke
Basil Bean, fava Asparagus
Bok Choy Bean, pole Bean, lima
Broccoli Bean, snap Comfrey
Brussels sprouts Beet Horseradish
Cabbage CarrotOkra
Cauliflower ChardParsnip
Celery CucumberPumpkin
Chinese cabbage Eggplant (aubergine)Rhubarb
ChivesKaleSquash, winter
Cilantro (coriander) MelonSweet potato
Collards PeaTomato
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!

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

1 comment on “Root Depth for Vegetables

  1. 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.

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