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Archive for May, 2009

Avid gardeners like ourselves have sometimes been stumped by the conflicting information you’ll get from different gardening pros and horticulturists.  Sometimes they’ll even contradict themselves.  Or worse, you’ll set up a companion planting scheme and discover that the insect repellent properties of, e.g. garlic to repel red spider mites from tomatoes, was completely ineffective in your particular garden habitat.

So we created a survey of plant companions from four sources to ascertain which combinations they generally agree/disagree on, with the understanding that observing and documenting our own results is really what matters in the end … though may not always apply every season!

Click here for PDF version of summary tables - copyright © 2009 Plangarden

Click here for PDF version of summary tables. © 2009 Plangarden

A PDF of the tables appears on the right.  More details are in the slideshow below this post. 

Here are some useful terms often used in the context of CP: allelopathy and phytotoxins. Allelopathy (allelo– “one another”, and –pathy “disease or suffering” from Greek) refers to how plants find ways to stake out their territory, often by harming nearby neighbors.  They may release phytotoxins (phyto– meaning “pertaining to plants”) which essentially attempt to knock out nearby competitors.   Phytotoxins can sometimes also repel insects and other animals.  

Some notoriously allelopathic plants harmful to the vegetable garden (though may be great at deterring pests which we’ll discuss in Part IV) are the eucalyptus, black walnut, and absinthe wormwood.  Make sure you know what lies just outside your property line if you decide to grow vegetables in that area.

Limited research has gone into subsoil fungal activity and its respective role in companion plants.  Some speculate that both beneficial and harmful compounds may be transmitted through the mycelial network – though it’s impossible for everyday gardeners to ascertain “fungal contributions” to the companion plant ecosystem.

In the end, what we suggest is to “not get overwhelmed” on which combination of plants will produce the ideal garden.  Our “strategies” post offers a starting point in figuring out what to grow.  Perhaps the initial approach in using these tables would be to scan those combinations least likely to yield beneficial results (the red circles), and then move on to the good companion pairings.

Is owning a book on companion planting an absolute must for the gardener?  Personally, we feel that you can get sufficient information to jot down notes from library books or from the Web.  However, if you love to experiment and collect gardening books, then look through the book reviews on Amazon.

In our next post, we’ll cover companion planting as it relates to the contribution of flowers, herbs, and insects to your overall vegetable garden.

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Now that you’ve had a chance to become acquainted with the brassica and allium families from Part I, let’s move on to often-overlooked areas in companion planting.

Other than light, watering, and feeding requirements, also take note of:

  • Potential crowding issues
  • Root depth requirements of companions
  • Planting poisonous companions
  • Companion planting as an art, not science

The term “companion planting” can sometimes lead one to thinking it’s OK to create a layout where companions must be in “close quarters” to each other.  Remember that, like you and I, even plants need their “space”, so always consult your seed package or a good gardening book to ensure that you give lots of growing room to your plants.  Spinach is often taken for granted with respect to its need for space (yes, we have bungled growing spinach in an area where it thrives – due to our ignorance!).  Ideally, 6 in. spacing is desireable to grow large, healthy leaves.

Newer gardeners will often plant indeterminate tomatoes like Early Girls too closely together.  Crowding not only affects yield, but also ease of harvesting as you maneuver around neighboring plants.   Ideally, it’s good to know the approximate root depth of your plant species (Ref. 1, p.79) and provide plenty of nutrients at different levels in the soil.  If you don’t give plants space that allows their roots to flourish, then yield may be impacted regardless of the companion effect.

Foxglove is not an ideal companion plant if young children frequent your veggie garden!

Foxglove is not an ideal companion plant if young children are frequent visitors (and grazers) in your veggie garden!

We recently came across a companion planting book recommending foxglove (Digitalis purpurea) as a companion plant to potatoes, tomatoes, and apple trees for its growth-stimulating and antifungal properties. But it failed to mention that this gorgeous plant, containing digitalis glycosides commonly used to treat congestive heart failure, can be lethally poisonous from ingesting any part of it – roots, stems, leaves, flowers, seeds, and sap.  The same author suggests larkspur (Delphinium sp.) whose leaves are poisonous to aphids, thrips (and incidentally, humans, as well).  So while many plants may be useful companions to your edible garden, be aware of their potentially toxic characteristics (Ref. 2, p. 172) !

Lastly, credible sources like Cornell University’s Dept. of Horticulture, enjoin gardeners not to hang on to every word found in a CP guide as providing foolproof solutions. Experts encourage us to frequently check our plants (go ahead, look under the leaves!) and document observations from our own plant combinations.

References:
Selecting a companion gardening book can be a daunting.  There’s just so much out there!  However, Anna Carr, author of Good Neighbors: Companion Planting for Gardeners, appears to have done the most thorough review of both the “traditional/folklore” vs. “research” basis of companion planting for the most common and even obscure garden plants.  She also offers experimental garden layout and plant combination techniques for gardening geeks to try out in our gardens.  Her book should be available at your public library or used at online book sites.  It is unfortunate that this book is not being updated with more recent scientific research, but the principles of CP still apply to this day.

  1. Carr, Anna 1985. Good Neighbors: Companion Planting for Gardeners, Emmaus, PA:  Rodale Press.
  2. Riotte, Louise1975. Carrots Love Tomatoes, Pownal, VT: Storey Publishing.
  3. Little, Brenda 2008.  Secrets of Companion Planting, Sandy, UT: Silverleaf Press
  4. Smith, Edward C. 2000.  The Vegetable Gardener’s BIBLE, North Adams, MA: Storey Publishing

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