Bioindicator Plants Can Help You Determine WHEN to Start Planting Your Garden
Planting charts and tables only get you part way there
If you are a long-time gardener, the lengthening daylight uncovers that itch to start digging in some dirt. I’ve already got a couple of little pots of seeds germinating because I can hardly stand the wait to be full-on gardening this spring.
If you’re new to gardening, that same nudge to start soon can bring out anxiety about when it’s “safe” to get the process rolling. Because who wants to put effort into starting a garden only to have a cold snap kill all your little green plant babies? Or spend the time and money to have transplants of tomatoes and peppers only to watch them droop and stall and fail?
Either way (or in between) the age old question of WHEN to start your garden leads to hours of staring at mind-numbing charts of planting dates, cruising through online chats, or consulting Google gurus.
Ironically, there is a much easier and more accurate way to figure that all-important WHEN, and it works no matter where you live. In fact, it might ONLY work where you live.
Because my timing and your timing are going to be completely different. And your timing, and someone who lives across town from you, might also be different enough to determine whether your efforts meet with success or failure.
So what’s this secret that the planting charts aren’t telling you?
Bioindicator plants!
Let’s talk about what bioindicators are and why they are the closest thing to fool-proof garden timing that you are ever going to find.
What is a bioindicator?
A bioindicator (aka biological indicator) is an organism that provides information about the environment that is otherwise challenging to measure directly. The presence/absence, relative abundance, or condition of the bioindicator lets you indirectly understand and infer the magnitude and direction of impacts in the system.
For example, I live on the Fraser River some 350++ kilometers from the ocean. I know that when I see seagulls flying over the river this far inland, that there are salmon in the river. I don’t have to go to the river, set nets or cameras, or consult a chart. The seagulls are a reliable indicator of a salmon run.
If you use Google or AI to investigate the term, you will be left believing that the sole function of bioindicators is to monitor for pollution and human impacts on the environment. And while one of the modern uses of the term may have originated back in the early 1900’s in relation to pollution impacts, the truth is we use this concept all the time to evaluate our ecosystem.
Ancient humans observed the ebb and flow of plants and animals through the seasons, and knew with certainty that the appearance of specific flowers coincided with the appearance of certain birds, fish, insects and mammals. Where I live in British Columbia, First Nations communities have detailed oral history that describe the way in which certain patterns and events indicate the starting point of when important foods become available. So this is once again a story of science catching up to what ancient people already knew and passed down from generation to generation.
How do bioindicators help us time our gardens?
If you spend any time outside, you know the indicators of spring well.
The days get longer.
The snow melts and the temperatures rise.
Trees start to have green buds.
Spring bulbs like crocus and snowdrops start to bloom (or if you prefer, focus on native plants like balsam root and meadow buttercup which are early bloomers where I live)
The grass starts to turn green.
Small leaves and spring blooms appear on the trees.
More plants start to have green leaves and flowers.
Spring blooming bushes start to flower.
And so on. . . . .there is a pattern to spring that is recognizable and familiar year after year.
What you may not understand if you are less familiar with plant biology is that there are a couple of mechanism that underly this sequence of events. One is the amount of light and the other is the temperature.
When it comes to timing your spring garden, we want to focus on plants that are triggered by temperature. We can trace the origin of “day degree modeling” back into the early 1700’s when a French scientist began predicting the timing and growth of plants based on temperature. This science of “phenology” has been evolving ever since.
You can visit the Oregon State University Extension Pages and get access to the most current day-degree models for crops (see Croptime here). You can Google day-degree models and find codes and apps that run other versions of this same concept.
OR, you can simply step outside and start observing the patterns happening in your very own backyard!
Plants do not generate their own body heat the way we do. Plants rely on their environment to accumulate heat and grow. Plants speed up and slow down their growth based on temperature. And for some sensitive plants, being to cold (or too hot) will kill them.
The plants growing in our own yards and neighborhoods are telling us about the exact light and temperature conditions they are experiencing. And as an added bonus, these plants are already accounting for any fluctuations happening locally as a result of unusual weather patterns rooted in climate change.
This is what make plants ideal indicators of WHEN you should plant your garden!
What are the best indicator plants to choose?
While you could start monitoring every bud and twig, you simply don’t have to. A few well-chosen bioindicator plants will give you the key transitions.
While I cannot easily tell you the specific plants for your area, I can give you the triggers you are looking for to match up your garden timing with local conditions.
The benchmark temperatures that impact spring garden success the most include:
The start of workable soil
The last frost
Daytime temperatures above 10oC (50F)
Nighttime temperatures above 10oC (50F)
Workable soil isn’t a plant bioindicator per se but it is critical to spring gardens. It simply means that you can dig into the soil surface and it crumbles easily. There are no frozen clods and the soil is not mucky or muddy.
You can hit periods of workable soil even though overall temperatures are cold and overnight freezing is still happening. If daytime temperatures are high enough, the soil surface will loosen. A quick hand-shovel test is all you need.
Just be aware of the difference between workable soil and warm-workable soil, the latter meaning that nighttime freezing is over.
Frost is a thin coating of ice that can form overnight when temperatures are between 0.5 and 2 oC (33-36F). Many plants and seedlings can be killed by frost, so it is important to be aware of this temperature range. However, other seedlings like spinach and kale can often survive mild frosts without damage.
Most of the cold-hardy spring vegetables that you direct seed grow well once the danger of frost has passed and daytime temperatures regularly climb above 10oC (50F). This is the range in which you may see perennial plants like rhubarb, asparagus and raspberry canes emerging. Herbs like chives will spike up through the ground. And most of the plants thriving in these conditions are going let you get a jump on food production.
Nighttime temperatures above 10oC (50F) is the fundamental range necessary for all the heat-loving summer vegetables. Choosing a reliable indicator for this benchmark is the ticket to a successful growing season.
Therefore, to use bioindicators effectively where you live, you are looking for specific plants that grow year after year and relate to these three important temperature shifts.
Good plants to monitor as bioindicators may include:
Spring bulbs like crocus, tulips and daffodils
Trees that produce catkins (wind pollinated cones) break dormancy faster than most other trees. These include poplars and birch.
Spring blooming shrubs like lilacs and forsythia (often coincide with the emergence of bees - who are another bioindicator)
Spring blooming fruit trees – apricots, apples and pears all bloom at slightly different temperatures
Spring emerging plants like rhubarb, asparagus and herbs like chives, where the plant dies back to the ground and re-emerges in spring
The timing of “full leaf” status of trees and bushes
This list just scratches the surface. It will be different in Arizona than Alaska. But as you become tuned into the sequence of plant changes that link to the benchmarks for good gardening, you can shorten your list to just a few plants that signal it’s time to put on your gardening gloves and get busy.
And while models are helpful for pointing us in the right direction, and may narrow down your list of indicator plants faster, the bottom line is that plants growing in your backyard are doing “the math in their heads” so to speak. They are automatically responding to the conditions they are growing in, and as such, they have better data than the models.
Want an even faster way to know for sure?
There’s one more trick every gardener needs to have up their sleeves for the best ever timing of their spring garden – use indicator vegetables!
So I have already mentioned rhubarb, asparagus and chives, but what if you don’t grow those?
Well here’s the ultimate trick for spring timing:
Sprinkle a small amount of cold-hardy vegetable seed (like spinach, kale, lettuce, mustard, or onion) on workable garden soil. Cover very lightly with soil. And watch what happens.
When these test seeds start to germinate, then you know the temperatures are warm enough to support growth.
If you are a seed saver, chances are you can skip the sprinkling step altogether and just look at where your lettuce or Pac Choi patch was from last year. Usually volunteer plants act as a built-in bioindicator that the garden soil has warmed up enough.
You can use this same trick again to time your transplants of warm season crops like tomatoes, peppers and squash. But I have found the best heat-loving indicator tends to be cucumbers. Cucumbers germinate faster than the other heat-lovers, and so once you see them go from sprout to a small cucumber plant, you are usually safe to start transplanting your more sensitive heat lovers. Just be sure to avoid transplanting shock when you do.
Bioindicators do the hard work for you
Garden calendars and seeding charts can only get you so far when it comes to knowing when to start planting your favorite vegetables and fruits. In the long run, you need to understand your very specific growing conditions to make gardening easy and successful. While this may sound like hard work, bioindicators actually do that heavy lifting for you.
Because we are, in fact, ancient humans living in a modern world. When we put our eyes and senses to work in the garden, we tap into our deeply rooted need to secure our own food. This connection to the natural world, can actively reshape our relationship with food, and can contribute to our wellbeing in ways far beyond the nutrition we are growing.
These observation skills also give us a front-row seat to the impacts of climate change where we live. I’ve already seen such strange things as spring blooming plants re-blooming in the fall, biennial plants producing flowers and seed in their first year instead of their second, and leaves not falling off deciduous trees, but instead holding on all winter long. These kinds of shifts should worry us all! These kinds of shifts should motivate us to do something about climate change before our food plants can no longer navigate the changes themselves!
While the news is busy shocking us with climate change scenes of wildfires and superstorms, the real crisis is going to come when our food plants can no longer survive in the conditions being thrown at them. Food for thought!
Choosing bioindicators to help you garden more successfully means that you will be tapping into the deep knowledge of plants and using that to generate food. This is the perfect time and focus for a garden journal, that can help you keep track of your observations and your planting-decision results.
Happy gardening!