Over the past decade there has been great interest in lavender production in Ontario and other areas of North America as agritourism becomes increasingly popular. I have been contacted by hundreds of people interested in growing lavender over this period. The conversations usually begin with one of the following:
I bought a farm. I want to grow lavender. What do I need to know?
I plan to add lavender to my farm. Where do I purchase plants?
I planted lavender on my farm. How do I manage it?
Most prospective lavender growers, including these, miss one key question for lavender production that will determine whether they have any chance for success: Is my soil suitable for lavender? With the wrong soil, successful lavender production will be very difficult.
Most people familiar with lavender have heard that lavender does not like wet feet. But what does this mean? Plants that do not like wet feet usually need good aeration in the soil so oxygen can get to the roots and the roots can penetrate easily through the soil. Without that oxygen, the root will die or will be weakened enough to allow pathogens to attack the root. This leads to poor top growth and poor winter survival.
Obviously continual saturation of the soil or periods of standing water are bad for lavender. However, this is not the only situation that can result in poor aeration of the roots. Smaller soil particles tightly packed together can also be poorly aerated, even when the soils are dry. The distribution of particles of different sizes in the soil determines the soil’s texture.
Soil texture can be tested by a laboratory to separate the soil particles into the proportion of sand, silt and clay. Sand particles are between 0.05 and 2 mm in diameter. Silt particles are between 0.002 and 0.05 mm in diameter. Clay particles are those less than 0.002 mm in diameter. This means that sand particles can be up to 1,000 times bigger than the largest clay particles.
The proportion of sand, silt and clay determines the soil type, which can be found by using the soil texture triangle (Figure 1). The triangle is used by lining up the percentage of sand, silt and clay. Truly sandy soils (sand and loamy sand), which are ideal for lavender, form a very small fraction of the texture triangle and contain at least 70% sand.
Lavender prefers sandy soils because of the large air spaces between soil particles, and the rapid drainage those air spaces provide. As soil types transition from sand to loam to clay, they become progressively worse for lavender production. Although those soil types can still have a proportion of sand, the smaller particles can fill the air spaces between the sand particles. Picture a large bin full of beach balls and the large air spaces between those beach balls. Now picture thousands of marbles being added to that bin. The marbles would fill all the spaces between the beach balls making no difference in the amount of large air spaces than if there were no beach balls in that bin at all, and it was all marbles. However, there are some other factors that may make heavier soils more conducive to lavender production.
While the proportion of sand, silt and clay in a soil (i.e. the soil texture) is vital for determining the success of lavender, how those particles are arranged in the soil is also very important. Soil particles can be bound together by several different processes including by humus in the soil to form larger aggregates called peds. Humus comes from the breakdown of organic matter and can act like a glue to bind the particles together.
Clumps of soil particles can behave like larger soil particles and allow for improved aeration, root penetration and drainage. Soil structure is improved by organic matter, a healthy soil ecosystem (e.g. worms, soil insects), minimal disturbance, and the use of cover crops. Soil structure is destroyed by excessive cultivation, cultivation when it is too wet, heavy rainfall on bare ground, erosion, and compaction with machinery or repeated foot traffic.
So, can you make a clay soil behave like a sand? Short answer is NO. You can improve it, you may be able to grow lavender with site modifications, but it will never be as good as a sandy soil for lavender production. Even if you can amend it successfully, what is the soil like underneath the topsoil? It is likely to still impede drainage even if drainage is improved in the topsoil layer. However, all soils are better for lavender production if they have good structure.
I have had many lavender growers ask about applying sand or even gravel to a clay soil to improve drainage and/or make it more suitable for lavender production. It sounds logical that adding sand would make the soil more porous but look again at the soil texture triangle. A clay soil contains anywhere from 40 to 100% clay particles. If you started with a soil with 70% clay, you would have to add enough sand to bring that down to less than 40% clay before it would behave like anything other than a clay. How far down in the soil profile would you have to amend? Lavender roots can go over a metre down in the soil. Would you be able to amend the soil to that depth? Here is an example of the practicality of amending soil with sand:
You have a clay soil with 60% clay, 30% sand, 10% silt. You want to make it a sandy clay loam that has 30% clay, 60% sand so it behaves less like a clay, even though it is still not ideal for lavender. You want to do this to 0.4 ha (1 acre) of land. If you added 10 cm depth of sand on the surface of the soil, you would raise the top 30 cm of soil to 53% sand; not quite enough, but an easier calculation. Sand to 10 cm depth over 0.4 hectares is 400,000 L of sand, or 400 m3 (523 cubic yards), with an approximate cost of $10,000 not including delivery and your costs for spreading it.
Remember what I mentioned about soil structure? In the process of incorporating that sand you would destroy any structure the original soil had resulting in compaction, changes to water movement, increased frost heaving over winter, poor drainage, and poor lavender root growth.
As it turns out, sandy clays are some of the worst soils, even worse than the original clay. Plus, the soil 30 cm down is still clay and rain water would still penetrate this layer slowly, resulting in a pool of water in the top 30 cm for extended periods. Watch this video to see what happens when rainwater hits a clay layer in the soil (start at 2:53 in the video).
What if you only put the sand in the row with the lavender? You would have the same problem with destroying the structure. The water would pool within that row and then penetrate slowly through the clay. Also, since the lavender roots go at least a metre in all directions, what purpose would it serve to just do a narrow swath?
What about gravel? Gravel is just like gigantic sand particles, and just like sand, the clay will just fill in all the air spaces between the stones and you will not have made any improvement to the behaviour of the soil. What do you get when you mix clay, sand and gravel? If the clay is limestone based, which most of them are in Ontario, then you basically have the recipe for concrete, and that is probably what you will get. Even worse, what happens when your lavender crop fails, or you move on to something else? Now those added stones and the destroyed soil structure significantly degrade the value of the land for other crops. Bottom line: DO NOT ADD GRAVEL OR SAND TO YOUR SOIL! Even putting a rock mulch on the surface is a bad idea because of the residual effects of these stones if they cannot be efficiently removed.
How to Fix Poorly Draining Soils
You cannot fix a poor soil for lavender production entirely no matter how hard you try. You can improve the structure over time with cover crops and addition of organic matter, but you will likely need additional measures to improve the site for lavender production. Here are some additional measures that can be done:
- Find a Slope. A slope is your best friend if the soil is not ideal for lavender production. Slopes allow for rapid surface drainage, preventing the soil from saturating too easily. Over the winter, slopes are good for lavender on all soil types because rainfall or snow melt onto frozen soil will pool, regardless of the soil type and could damage the lavender.
- Install Tile Drainage. A drainage tile placed under every row of lavender or throughout the field can help remove excess water rapidly after a rainfall and in the spring. This will work best if there is at least a gentle slope so surface water doesn’t pool on the surface before it has a chance to infiltrate to the drainage tile.
- Plant on Raised Beds. Large raised beds will allow surface water to run away from the crown of the plant and elevate some of the roots out of any saturated soils. They would not be needed if there is already a good slope. They need to be large (e.g. 1 m wide, at least 20 cm high) or they will erode too quickly and not contain enough roots to make a difference. The beds will need to be covered with plastic or other ground cover to prevent erosion over time. Water still cannot be allowed to sit for lengthy periods between the rows, so raised beds work best when combined with gentle slopes and/or tile drainage. In forming the beds, you may also degrade the soil structure, so do it with as little disturbance as possible.
It is always best to try any new crop on a small area of your farm to make sure it works before investing too much. Once you can make it work on a small area, then you can consider expanding it. There are some realities that you will need to face: your soil may not be suitable for lavender production and there may be nothing you can do to improve it. The good news is that heavy soils can be excellent for many other crops. Consider growing something else.
The Take-Home Message
Sandy soil: you are good to go for growing lavender on your site.
Loamy soil: you can probably make it work with good soil structure, improved drainage and/or a slope
Clay soil: You may be able to grow lavender if you have the combination of good structure, tile drainage and raised beds and/or a slope, but even then, there may be problems with root disease and overwintering. Try it on a small area first.
Adding sand or gravel is usually not practical and will likely make the problems with your soil even worse.
Keep in mind that even with good soil, lavender is marginally hardy in Ontario and winterkill can still occur. Climate and microclimate management is the second key to successful lavender production.