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This last section on companion planting (CP) sums up practical ways to cultivate a healthy garden with some “built-in” natural protection.

Intercropping vs. monoculture

By this time, you should be convinced that intercropping to create a diverse vegetable garden “habitat” is most effective to naturally protect your garden from pests, and can potentially enhance the yield of your favorite crops. Monoculture not only makes your valuable veggies more susceptible to pests in one season; sometimes, despite your efforts to work in rich, organic fertilizers, the same family of plants grown in the same area will fail in later seasons.

We learned this the hard way as we were initially concerned about having too many different veggies with too little yields.  As it turned out, yields started falling in later years as our broccoli became more susceptible to pests (BTW – brassica family, unlike tomato plants, generally like to be moved around).  So be daring, be bold, and experiment with different veggies – even those you think you’d never like!

Exploiting garden warfare

Your lovely spring garden or the woods near your home may look “serene”, but in truth, there’s constant warfare going on at a physical and chemical level.  So why not take advantage of it?

For instance, sunflowers may be great weed deterrents as they were found to be strongly allelopathic to weeds like wild mustard, jimsonweed and ragweed (Ref 1, p. 310).  Or what about selecting a “sacrificial plant or soldier” to protect the more valuable crops?  For instance, kale can be planted at borders of other more valuable cabbage family plants to draw pests away.  Or black nightshade can be used as a decoy plant to attract Colorado potato beetles away from your potato plants.  This may not be the best option for those of you with tiny gardens, but try to think of creative ways – even using containers which can easily be moved around – in deploying “soldiers” to protect your cash crops.

And while we’re on the subject of war, keep in mind that CP is not necessarily a mutually beneficial combination.  Some gardening experts believe that it is almost one-directional, though this can be difficult to ascertain.  For instance, you might consider that if carrots were planted next to peas or tomatoes (as our comparison of CP guides suggest), would yields from peas or tomatoes be greater at the expense of the carrots?

Using ornamentals to attract beneficials and help control pests

Many gardeners certainly deploy ornamentals like nasturtiums and marigolds to protect plants from pests like aphids, detrimental nematodes, white flies, and other pests.  But having herbs and flowering plants (esp. daisy and parsley families) also benefits the vegetable garden by attracting beneficial insects and birds that eat nasty bugs or pollinate plants.

We have had success growing tomatoes and beans with marigolds in containers, but have yet to attribute the yields/health of the plants to the marigolds. Some CP guides will suggest different varieties of marigolds for different pests (African, French and golden marigolds appear to be the best varieties).

Nasturtiums (whose flowers are a mildly peppery but delicious and attractive addition to your salad!) do have to be kept in check as they can take over an entire area, and are an incredible snail magnet on the West Coast.

Recording your own companion planting results

If you’re really intrigued by companion planting and want to put it to work for your garden, then record your observations.  Your best CP guide is your own gardening experience, though keeping in mind that results may not always be reproducible (see Part I).  A simple notebook or spreadsheet works perfectly well (of course, there’s Plangarden, too 🙂 )  Some variables to keep in mind are the yield and planting distance in the control vs. experiment plots.  For the die-hard garden geeks, we refer to Anna Carr’s excellent book (Ref. 1, pp. 332-355) that offers experimental techniques.

Good luck and may the companion forces be with you!

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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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For centuries, people have observed beneficial and detrimental relationships among plants in specific configurations.  Companion planting (CP) is all about cultivating vegetables and flowers that will be beneficial to your plants and ultimately, enhance your harvest.

Know Your Plant Families for Companion Planting

Know Your Plant Families for Companion Planting

But because of the complex interactions among plants, animals, the chemical composition of the soil and air, and even extraterrestial influences that some gardeners swear by like the phases of the moon, certain guidelines are more consistent than others.  In many respects, edible gardening is an art that we in our unique locations must tailor for our own ecosystem.  We need to draw on our powers of observation and common sense if what was prescribed did not produce the desired results.

Generally, companion planting addresses various objectives such as:

  • Yield enhancement
  • Natural pest control
  • Attracting beneficial insects
  • Watering requirements

What you’ll discover in doing a Google search is that there are CP guidelines that are consistent across different environments (i.e. may have a valid biological/chemical basis), and then there others that are “hit or miss” (i.e. may have originated from anecdotal evidence subject to conditions that cannot be duplicated in most environments).

To understand companion planting, it helps to know your basic plant families.  We created a Garden Vegetable Family slideshow to acquaint you with the basics, so that if you see the term “nightshade”, you’ll immediately think of tomatoes, potatoes and bell peppers, and how it’s often not recommended to plant them near each other.

Our next blog entry will talk about the common blunders in companion planting.  While there is no shortage of companion planting books and guides, our objective is to provide guidelines that are almost universally agreed upon — with this thoughtful caveat:

“Experienced gardeners will commit themselves to few rules and even fewer certainties with companion planting.  Most of us need to experiment with several approaches and will usually discover that success depends upon location, soil condition, and other factors in addition to the combinations themselves.  Bearing this in mind, it is good practice to keep detailed records of each attempt so that similar conditions can be repeated whenever a successful outcome is achieved.”

Derek Walton, http://www.organicguide.com

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