Canary Plants as an early indicator of soil health

This was a musing from the Soil week of my Permaculture Design Course

We were talking about Soil this week in my Permaculture Design Course with the wonderful Ian Epic Earth, and I learned so much.

As we were chatting about mineral and water deficiencies and how they show up and can be identified and diagnosed on leaves and other parts of the plant, someone in the course mentioned “When I see my lemongrass looking all droopy and sad, I know it’s time to water the whole garden.” And it got me thinking about the idea of ‘canary plants’.

What is a Canary Plant?

The term canary comes from the literal bird, and their historic use in coal mines. You’ve almost certainly heard the saying “a canary in a coal mine”. They are very chatty songbirds, but very sensitive to toxic gases, like carbon monoxide and methane. So for a long time (almost 100 years between the 1880s and the 1980s) they were brought down into the mines, and their death (or silence) when they stopped singing was a clear indicator something was wrong and the miners were in an unsafe situation. It affected the canary much quicker than it affected the miners.

Which brings me to my idea of canary plants - I guess a quick google search will deem if this is my unique idea or it already exists. Please Hold…

Okay, it seems like a novel term!! The only response relevant I got back was from Google’s AI and not very helpful.

My idea is to plant specific plants in your garden for the purpose of signalling you clearly, quickly, and definitely if an issue needs to be addressed. This allows quick remediation of soil deficiencies, across the whole bed - allowing you to mitigate issues before plants are even showing significant stress from the cause. Similar to a trap crop, I guess you don’t have to eat or use your canary crop - it can be sacrificial, but I reckon you can eat it too!

Defining a Canary Plant

So my criteria for a canary plant would be:

• Shows very early warning signs of an issue - more sensitive to it than the average plant I am growing.
• Is highly diagnosable - based on the plant behaviour, I can easily and definitively diagnose the issue.
• Solving the issue benefits the whole garden around it - For example, I’m not overwatering or over-fertilising the plants around it. This would require plants with similar needs but higher sensitivity. I’m hoping I can find a decent proxy here for an average veggie garden.
• Demonstrates recovery if conditions are improved - let’s me know I’ve solved the problem.
• Bonus points for
  • Multiple issues diagnosable by the same plant and clearly distinguished - means less canary plants required.
  • Edible or useful in another way (ideally another 2 ways) beyond being a canary.

Candidate Plants:

• A great one in an indoor plant indicator of low moisture is the peace lily
• For now, I’m going to focus on annual veggie beds and the classic things grown there and something that could signal for those plants.

Existing Research

Okay turns out these are called ‘sentinel plants’ and there’s been a few research papers on them:

A special tomato

• this research institute has developed a line of sentinel plants that can serve as early reporters of field issues, allowing farmers to take preventive action before it is too late.
• The CEPD:RUBY tomato is unique because it reacts to nitrogen deficiency by turning its leaves red.
• They recommend 15% sentinel density in broad acre farms.

Plants to trap pests

• The concept of sentinel gardens in NZ is also used in the context of pests - in terms of attracting dangerous/invasive pests and monitoring them so they can be reported to DPI

Potential Veggie Garden Canary Candidates

Cucumber

• Wilts dramatically in low water / drought conditions.

Sweet corn

• Nitrogen, Potassium, Phosphorous, Sulfur based on leaf pattern and if its old or new leaves showing the problem
• It does take up a lot of space though - could grow a small/dwarf sweet corn variety at the southern side of a bed
• Particularly good at N deficiency diagnosis - yellow V running from leaf tip down to the midrib starting on oldest/lower leaves
• Particularly good at Potassium deficiency diagnosis - yellow/brown on outer margins of older leaves while centre stays dark green
• Also helps diagnose Zn deficiency with broad white/pale band on either side of the midrib on upper/newer leaves
• Go with a standard sugary su sweet-corn > supersweet sh2

Large/paste tomatoes with blossom end rot - unstable water/Ca delivery to fruit - dark sunken, leathery patch specifically at the blossom end

• susceptibility varies by tomato type, with larger plum/paste tomatos more susceptible, while cherry toms etc rarely show it.
• Would need to select a variety that is a blossom end rot sensitive cultivar
• subsequent fruit can develop normally, which gives a remediation feedback loop
• Would be good to use a moisture meter when we do this to run it as an experiment
• tensiometer might be a much better instrument than a plant here - measure water tension
• Chinese cabbage getting inner/new-leaf tipburn in winter.
• Pair this with a tensiometer to be suss that it might be a calcium issue is moisture is okay and has been consistent.
• elongated pate/roma types - Roma/Roma VG (plain roma)

Idea for an experiment for this:

• 2 beds side by side, one with the sentinel and doing the amendments, and one without at all?

Winter wheat - a winter version of the corn N and K gauge

• N deficiency - Yellowing on oldest/lower leaves beginning at the tip and progressing towards the base (inverted V)
• Speckled / mottled oldest leaves - centre remains green
• maybe RGT Healy as it’s known to be N/K sensitive but less disease prone in our climate