What are growing degree units?

Last week I teased you about growing degree days.  Weren't you intrigued and just dying to know more?  Probably not, but that doesn't matter because you're going to find out.

Let's start with the problem.  I have two greenhouses set at different temperatures.  I also planted greens on different dates throughout the fall.  In the chart above you can see:

1. The average temperatures were different in the two greenhouses, especially as it got cooler
2. The temperatures vary depending what time of the season greens are planted 

This is where growing degree units come in!  But what are they?

"Growing degree units are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and pest development rates such as the date that a flower will bloom or a crop reach maturity."  Wikipedia 

So, they measure heat accumulation.  Luckily they are easy to calculate, you just need to know:

1. Average daily temperature
2. The base temperature, below which your plants won't grow.  For salad greens I use 40 degrees F.  This is called the biofix.

With these two things, and the date you can calculate the number of growing degree units in a table like the one below.  All you do is subtract the biofix from the average daily temperature. The difference is the number of growing degree units accumulated that day.  Then you can add them up over a period of time (like a growing season) and compare differences in temperature!

These two tables show the number of growing degree units accumulated in each greenhouse over a ten day period.  Greenhouse 2, the warmer one, accumulated nearly twice as many growing degree units in this time!

Growing degree units are used to predict flowering blooming, crop maturity or insect emergence.  I've been using them to examine the growth rate data, but more on that in the future

Source

Two very cold greenhouses

Last Friday I went to check on the greens and see if they needed to be watered.  In the winter they don't need much water, so I don't go in as often.  When I went in the greens looked wilty - wilty? Even though it was below freezing outside?  Closer inspection showed the media was frozen on top and the greens were not wilted, but frozen.  Here's what happened in a temperature timeline:

Fall fuel use

While sifting through all the growth rate data I collected last fall, one thing I'm looking at is fuel use, and if using more fuel to speed up growth rate is cost effective. 

The greenhouses are heated by a furnace that blows hot air out (left).  They have a meter on them (right) that shows how many hours the blower has actually been blowing air out.  I record the number twice a week.  Since the meter is continuous I adjust the numbers to reflect the number of hours it was on for my particular experiment.  Oh-so-conveniently, the furnace uses one gallon of fuel every hour, so I also convert "hours" to "gallons."

The above chart shows total fuel use in each greenhouse.  Greenhouse one is heated to a 40 degree (F) minima and greenhouse two is heated to a 50 degree minima.  This doesn't mean that each greenhouse is at a constant temperature, it means the temperature will not fall below the given set point.  You can see this created a big difference in how much fuel was used. 

Since 'gallons of fuel' is a bit arbitrary to the non-greenhouse owner, lets attach monetary values (everyone understands dollar signs). So,

Greenhouse 1: 108gal x 3$/gal = $324

Greenhouse 2: 264gal x 3$/gal = $792

That is a big difference!  But is it economically worth it to heat the greenhouse to a higher temperature?  Will that make greens grow faster?  Or is it just throwing money down the drain?   That's what I'm working on figuring out (but haven't done so yet!).

* As usual, this data pertains only to our greenhouses this fall.  It is not complete and should not be used elsewhere.  If you are looking for data pertaining to winter greens growth, fuel use and temperature you'll just have to wait until I'm done with my thesis! *

A few things I've learned this week

I have spent much of the past week writing and organizing for next fall.  So, things are happening, but I don't have anything exciting to show for it.  (I don't think you'll find eleven pages of scientific writing as exciting as I do.)

In gathering together notes and papers I've been collecting for the past year I've come across some cool facts about food, and food culture.

#1.  Spinach is my all time favorite green.  Harvesting it is a labor of love because each leaf has to be cut individually.  But, it will grow, as in make new leaves, at only 40 degrees and those leaves will be 2 brix degrees of sweetness higher.  What is a brix degree you ask?  The grams of sugar dissolved in 100g of solution.  I don't really know how much a gram of sugar is or how the brix scale actually works, but I do know that winter grown spinach is noticeably sweeter.  Some that I grew last winter was like candy it was so sweet and that is a good thing.


#2.  The average person spends $20 at a farmers market.  Hopefully this is not the only thing at which I am above average.


#3. During winter months much of the produce consumed in the United States is grown in Arizona or California and transported to consumers via truck.  In a Michigan study, locally grown winter salad greens grown in low and high tunnels use less energy to produce than greens imported from warmer climates.  

Sources: #1, #2, #3

Growth rate research questions

Since my growing season is over, and I no longer have any plants in the greenhouse, most of my time is spent organizing data, writing and trying to answer all my research questions.  I have lots of questions.  The ones below pertain only to the growth rate of greens and the economic of growing them because that's what I am thinking about right now.

Germination trials: Recap

It's come time to wrap up the Germination Trials so we can move on to the next experiment before the winter season is over.  I won't finish analyzing the data until this summer when I have more time, but  there are a few conclusions that can be made now:

 1.  The different treatments did have an effect on % germination rate and days to germination for all three varieties of seeds.

 2. The three species of greens responded differently to the cool, wintery conditions in terms of germination rate. 

3.  How all the temperature and light data tie into the germination rate will have to wait until I have more time.

As with all experiments, new questions for further experimentation arise.  The questions I'd like to explore in the next experiments are:

• How do the tunnels affect growth rate?  I showed that there seemed to be differences in growth post-germination.  Now it's time to actually measure it.
• What about other species?  We only looked at three species of greens.  What about green leaf lettuce, arugula, kale or tat soi?
• Would combining treatments be advantageous?  Putting heat mats inside clear plastic tunnels seems a good combination to provide root zone heat, above ground heat and a humid environment.
• What questions do you have?

Greenhouse temperature

Today is graph day!

This week I have been thinking about temperatures.  If you remember back to when I set up the temperature probes there would be thousands of temperature readings.  All that data in a giant, overwhelming spreadsheet taunting me because I am not so sure how to use it all.  While figuring that out I've been looking at just a few days at a time.  Kind of like snapshots.

The greenhouse temperature itself is a good place to start. 

Soil temperature in control treatments.

I realized after I made the above chart that I had used soil temperature data instead of air temperature.  See what having oodles of numbers in a huge spreadsheet does to me?

Below I graphed soil and air temperatures together.  You can see the treads I highlighted in the soil temperature graph can also be seen in the air.  The major difference is air temperatures are much more volatile.  They seem to jump up or down a few degrees each hour.  This could be due to cloud cover, or air movement from the fans.

Soil and air temperatures in control treatment.

*  This data represents five days in the greenhouse I am working in.  

Other greenhouses or high tunnels may have similar temperature trends but they will not be identical.  *

Seeds are sprouting!

Last week I did a second planting of lettuce, spinach and mizuna.  I spread the work over three days, so it was far more relaxed than the first planting.

Now, 12 days after being planted, lots of seeds are sprouting.  For each tray-full of seeds I record the day each seed sprouted in a table like this:

Like before the green cells are lettuce seeds, the orange are spinach and the blue are mizuna.  So far the seeds are sprouting in the order we thought they would - first mizuna, then lettuce and finally spinach.

Germination trials: measuring light

How much light is actually reaching the inside of the greenhouse?
How much light gets through the low tunnel coverings?

To answer these questions we use watch dog weather trackers.  They measure:

    ~ average daily temperature

    ~ daily high and low temperature and the time at which they     

       happened

    ~ amount light in moles/day

Pretty useful little things.  

Germination trials: measuring temperature

The big fancy data logger from onset.com

One of the major components to this first experiment looking at germination rate is measuring the temperature in each of the treatments.  Happily there are data loggers that continually do this for you.  We are using two models from Onset: a big fancy looking one and a plain box one.  The fancy one is more expensive and is water proof so if I spray it with a hose by accident it will be ok.  The smaller one isn't water proof so I installed it way up high where only the hose shouldn't reach.

The small not-so-fancy looking data logger from onset.com

Both data loggers support four temperature sensors.  For the germination trials we need two sensors per treatment for a total of 24 sensors on 6 data loggers.  The data loggers can be programmed to take a temperature reading at any interval you would like.  We are going to set them to measure the temperature every hour on the hour.  In one week there will be over 4,000 temperature readings!  All this data is downloaded into a software program from Hoboware.

The picture above shows how the data is displayed with Hoboware software.  Each line, or dotted line, shows a temperature probe over a five day period.  Hourly temperature readings can be scrolled through or exported to excel.  

Germination trials: low tunnel coverings

Once the PVC pipes were set up on the greenhouse benches it was time to cover them, creating mini-greenhouses.  This led to an introduction into the world of plasticulture.  Tons and tons of plastic is used to grow our food.  So much, that the word plasticulture is used to describe all the greenhouse coverings, drip irrigation, mulches, wind breaks, post harvest handling, marketing, and fertilization tools used to grow food crops.

There are a lot of different kinds, colors and weights of plastic to use when making a greenhouse, high tunnel or low tunnel.  Here are the four kinds of plastic we picked and one non-plastic treatment:

1. Sun Master 70% Opacity overwintering film

From it's name you can tell that not much light gets through this white polyethylene plastic.  It's the kind of plastic that is put over greenhouses or tunnels that house woody plants in the winter to keep temperatures within from fluctuating too much.  So, it should be "warm" inside which will promote seed germination, but it will not be good place for leafy plants to actually grow.

2. Sun Master Greenhouse Film 

This is the clear plastic that you would expect a greenhouse to be made out of.  It is 6mm thick polyethylene and allows a whopping 92% light transmission through.  So, this one should keep it "warm" inside as well as let plants grow because they will have some light.  (I say some light because all the layers of plastic add up.  If there are two layers of plastic on the greenhouse itself and one on the low tunnel only 78% of the available light can be transmitted through.  On a dark winter day that isn't much light.)

3. Agribon+ AG19 Row Cover

This is a agricultural fabric but it made from plastic as well.  Polyester and polypropylene strands are bonded together in a swirly random fashion resulting in the term "spunbonded".  This makes a cloth like covering that allows water through, but doesn't actually absorb water as a true cloth would.  Spunbonded fabric comes in a range of weights.  The one we're using allows an 85% light transmission and weighs 0.55 oz/yd.

4.  Clear plastic with holes in it

I haven't sourced this plastic so I don't know much about it yet.  It is far thinner than the other plastics we chose so presumably more light will be transmitted through.  It is also covered in holes which allows better air flow.

5. KEN-BAR agritape heat mats

These obviously are not a covering, but they are the final treatment in this experiment.  They still alter the plant's environment by keeping the soil and roots warm.  Heat mats come in all sorts of shapes and sizes.  This brand often is used by people who keep terrariums filled with snakes and other cold blooded animals that I wouldn't want to have for a pet.  Right now we have the heat mats set to be 40 degrees Fahrenheit at night.













6. Control

Every experiment needs a control.   I forgot to take a picture, so you'll just have to take my word for it that there is a bench with a bottom layer of plastic and nothing else.

Germination trials or an overview of Experiment Number One

The first thing we are going to be looking at are seed germination rates in different low tunnel treatments within the greenhouse.  The goal is to create a low tunnel that will significantly decrease the time it takes for seeds to germinate.  In cold weather seed germination can sometimes be a limiting factor in plant growth.  For example, lettuce will germinate best around 68 degrees Fahrenheit, but will happily grow at much lower temperatures.

Here's an overview of what is going to happen:

The six low tunnel treatments

1. Clear plastic - the thick stuff used to cover greenhouses
2. White plastic 
3. Clear plastic with holes - a light weight plastic with holes that allow for ventilation
4. Floating row cover - often called 'remay' on farms
5. Heat mats - to provide bottom soil heat
6. Control - just plants in the greenhouse

The three varieties of greens

• Spinach - the slow germinator
• Lettuce - the mid-rate germinator
• Mizuna - the fast germinator

Why only these three greens?  To properly conduct a Scientific Experiment there have to be lots of replications.  For instance, if we do six replications of 24 plants per treatment that ends up being almost 2000 seedlings to plant and count for each species of green.  To keep the counting low-ish we picked three greens that represent a range of germination from fast to slow.

An abridged protocol

1. Finish building the low tunnels.  This involves drilling into metal using tech screws.  I am not good at this and have been putting it off.
2. Plant a lot of seeds.  They haven't arrived yet which is why I can put off the drilling.
3. Count the number of seeds that have germinated every day for ten days.  

That's it!  I'll be writing about the process and results as they unfold with plenty of pictures.

why fall is a bad time to start vegetables

I am getting all excited about growing lettuces and new varieties of greens that I haven't seen before but now is just is not the time.  It is still warm enough out but the real issue is lack of light.  At this time of year - late fall - five o'clock comes around and it is dark as midnight.  This affects plants just as it affects people.   Less than ten hours of sunlight significantly decreases the amount of growing leafy greens will do.

These little spinach plants illustrate this idea.  They were planted over a month ago and have a nice warm growing space inside a cold frame but grow very slowly because day-length has been decreasing.  Combating this dilemma is simple but requires a bit of planning (or planting!) ahead.  Greens and some other vegetables planted in late August and early September will have had ample time to reach maturity by now and will continue to "hang out" in the garden for a long time even when days are short and the nights get chilly.

Kale is a perfect example of cool hardy vegetable that will live in the garden.  Just make sure you plant extra at then end of summer.   Most cool hardy vegetables can survive light frosts.  Further protection from cold frames and row covers can help stretch the season a bit longer.  Cooler temperatures increase sugar and nutrient levels in vegetable making them even tastier.

plant profile: spinach

Spinach is my favorite green.  


It is not fast growing like some of the brassicas.  Nor does it have sexy colored viens like swiss chard or red russian kale.  Harvesting spinach can be a true labor of love when each leaf has to be cut individually.  Despite all of this, spinach is great.  No, it's beyond great - it is one really cool plant.  Here's why:

~ Spinach is a diecious crop meaning there are male and female plants.  In the absence of any male pollen producing plants female flowers can ‘revert’ to become pollen-producing flowers, enabling continuation of the sexual reproduction cycle.  

~ Spinach is one of the hardiest leaf crops. It grows best in sandy, porous soil but can adapt to a wide range of soils.  Spinach germinates best in cool soil and can germinate at temperatures as low as 35 degrees Fahrenheit.  

~ Spinach can grow at very low temperatures.  Other leafy crops can tolerate the cold but will not grow.  Spinach grown in the cold has higher concentrations of nutrients like vitamin C as well as higher sugar levels.

building the low tunnels

We have two kinds of benches, wood sided and metal sides.  

The wood ones are way easier to drill into, but either way setting up hoops on them is pretty easy.  

We used:

9' long pieces of 1/2 inch PVC

metal brackets

tech screws or metal to wood screws

The bottom of each bench is covered with a layer of clear plastic since our goal is to build a sealed chamber.  The plastic is a bit brown because it had been used once in the field.  Next we covered each tunnel with a different kind of cloth or plastic to see what effect they would have on internal temperatures.  Each tunnel has a temperature sensor that measures temperature on the hour so all we have to do is sit back and wait.