Creating a Balance : Vegetative vs Generative Growth

Part 2 - It's All in Your Water

There are many factors that play a role in managing vegetative and generative growth, and it is important to consider a well-rounded approach when attempting to swing the scales. Keeping a balanced growing environment is important for a highly productive plant and changing one thing and nothing else will end up causing more harm than good.

In part 1 of this mini-series, we addressed the need for vegetative growth to have a high yielding crop. Vegetative action is required early in the crop cycle so the plants can establish themselves. By creating this strong base, the plant will be able to support a highly productive generative state later on.

The generative growth phase focuses on managing plant productivity. It is achieved through strategic stress induction in your plants. These minor stresses are going to push the plant to divert resources to reproductive organs (fruits), while ensuring that all other plant processes can be maintained. The key word here is minor – pushing the plant too far is not going to have the response you are looking for.

How Do I Manipulate the Plant?

A few of the major contributing factors to vegetative and generative growth are listed here below. We will address the impact that each factor has on crop steering.

•        Water availability
•        EC
•        Day/Night temperatures
•        Vapour Pressure Deficit (VPD)
•        Pruning
•        Fruit load

Water Availability

Watering frequency, duration and timing plays a role in pushing the plant towards a more vegetative or generative state.

In vegetative action, water is readily available for the plant when it needs it. The plant does not have to work hard to uptake water and will readily create new above- and below-ground biomass. As roots and leaves are being produced, the plant is preparing for full-blown production. Higher water content in the substrate is achieved through small, frequent irrigation events throughout the day.

Once it is time to switch to generative growth, these watering events become longer and more infrequent. As water becomes less available, less energy is put towards the production of root and shoot material, and more will be diverted to fruit production. The plant will now have to actively seek water compared to it being readily available in the vegetative stage.

Figure 1. Depicted here is the general irrigation strategy for substrate-grown greenhouse crops (ex. Tomato). Water content (WC) is depicted in green. While the timing and frequency of watering for generative and vegetative phases will vary, the general concept applies. As the day ramps up, you slowly increase the amount of water you deliver to the crop. After you achieve first run-off, you can start hitting the crop with water at regular intervals (determined, in part, by the weather that day). As the end of the day approaches, irrigation stops all together. Photo credit: Greenhouse Canada, June 2010 (https://www.greenhousecanada.com/september-2010-2418/)

While the daytime irrigation strategy is important in crop steering, you must also consider what is happening through the night. The length of the dry-down period of your growing media will help to tip the scales towards vegetative or generative tendencies. A short dry-down period (usually achieved by scheduling the last irrigation event of the day 1-2 hours before sunset) encourages vegetative growth. A longer dry-down period (which would have the last irrigation event happening earlier in the day) will encourage generative tendencies.

EC Measurement

The electrical conductivity (EC) of your fertilizer solution is important to consider when encouraging vegetative or generative action. EC is an indication of the salt content in the solution and will therefore impact water availability to the plant. A higher EC value means a higher salt content, and the harder the plant must work to take up water. As the plant works harder to take up water, less resources will be sent to vegetative growth (leaves, roots) and more will be sent to the generative tissue.

Here is a rough guideline of where your target EC should be, based on the crop stage:

Plant Stage	Target EC
Germination (vegetative)	0 - 1
Plant raising (vegetative)	2.5 – 3
Harvesting (generative)	2.75 – 3.5*
Full harvest (generative)	2.75 – 4*

*Should be watering with an EC of around 3, but you will see a higher substrate EC as the salts accumulate in the grow media

Understanding how your watering habits sway the crop in one direction or the other is a good starting point. Irrigation is something that all protected producers have control over, and it is important to see that watering plays a huge role in plant phenology and overall production capacity.

Creating a system that encourages maximum production capacity is in everyone’s best interest, and it is extremely important that all factors are addressed when trying to push for a more generative crop. Adjusting the watering and EC alone are not going to be enough to hit the target yield. All plant processes are intertwined and should be carefully considered whenever adjustments are made.

Stay tuned for our next post on vegetative vs generative growth, which looks at the role of temperature and humidity in crop steering.

Posted by: Talia Plaskett

Creating a Balance: Vegetative vs Generative Growth

Part 1 - Setting the stage 

Most greenhouse vegetable producers will have come across vegetative versus generative growth in their vine crops (tomato, pepper, cucumber). Over the next few weeks, we will be exploring the differences between the two types of growth, what they look like, and how we can manipulate the plants to encourage one form of growth over the other. This information will be focused on tomato production, but it is important to note that the same concepts apply to other crops.

Vegetative growth is characterized by:

• Thick stems
• Lots of leaves and suckers
•  Leaves are long and can be twisted/curled
• Dense growing tip
• High density of leaves
• Small fruit
• Elongated fruit truss

The plant is putting most of its energy towards the production of plant material. While leaves are important for photosynthesis, tipping the balance towards a more vegetative state does not leave enough energy being diverted towards fruit production. The size and quality of the fruit coming off a vegetative plant will be significantly less than what you would want.

Figure 1. An image of an overly vegetative tomato plant. Notice the thickness of the stem, the high density of plant material, and the curling of the leaves. Photo credit: Government of Alberta, Tomato plant propagation in commercial greenhouse tomato production (https://www.alberta.ca/tomato-plant-propagation-in-commercial-greenhouse-tomato-production.aspx)

Generative growth is characterized by:

•       Thin stems
•       Fewer leaves, typically shorter in length
•       Good sized fruit
•       Slight purple tinge to the growing tip

These features indicate that the plant is putting most of its resources towards fruit production. While we want to see generative growth, it is possible to tip the balance too far. A plant that is overly generative, will not have the leaf resources available to power plant growth. Overly thin stems with few leaves will not be able to support a high fruit load, and the resulting fruit will not be of market quality.

 

Figure 2. A tomato plant that is growing generatively. Notice that the stem is significantly thinner here compared to Figure 1. The leaves are much less densely packed, and the leaves are not curling excessively. Photo credit: Talia Plaskett, Perennia.

Production of greenhouse crops requires both types of growth for optimum production. The act of rooting after transplant is considered vegetative. You want to encourage the plant to establish a good strong base in the growing media, so that it can support the plant through the growing season, and push for high yields. After a certain point, you want to start pushing the plants to a more generative state. This will encourage more fruit, and of a higher quality as well.

How do I know where I stand?

Designate one or more areas within the greenhouse as being your primary monitoring spot. This area should be representative of the growing conditions in that space – many growers will choose the centre of the tunnel/greenhouse, and pair this with the location where they are collecting information on drain EC/pH and slab weight. Each production space should be considered its own entity, as temperature, humidity, air circulation etc. will vary between bays despite our best efforts.

Within this monitoring block, a series of measurements should be taken weekly. Things like stem width (measured below the youngest flower cluster with an open flower), internode length, leaf length, weekly growth, and number of leaves/clusters will let you know how the crop is doing. Keeping this information on record will help you get an idea of how your interventions paid off, and generally how the crop is doing. These measurements can also be used to forecast future yields and therefore what will be available for retail.

Soil salinity in Nova Scotia high tunnels

When evaporation from the soil surface and transpiration from plants exceeds water input, salts can start to accumulate at the soil surface. This is because when water from the soil surface evaporates, it wicks salts that are suspended in the soil solution to  the soil surface.  This is particularly a problem in high-tunnels that are in production year-round.  

Accumulating salts are not just sodium (Na⁺) and chloride (Cl^-) ions, but are also essential plant nutrients... calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), ammonium (NH₄⁺), hydrogen (H⁺), phosphate (HPO₄^2-), sulfate (SO₄^2-), and nitrate (NO₃^-) are all salts. Nitrate accumulation is particularly frequent in high tunnels as high-value crops are well fertilized, and since there is no rainfall to leach the nutrients, they accumulate year after year.

To learn more about soil salinity, how it happens, and what you can do about it, please read Perennia’s factsheet: http://www.perennia.ca/wp-content/uploads/2015/12/Soil-Salinity-NS-Hi-Tunnels.pdf 

Pruning cherry tomatoes

The Northern NY Agricultural Development Program did a trial on pruning efficiencies in cherry tomato production.  There were three treatments:

1.     Single Leader (one leader, all suckers removed), spaced at 12 inches
2.     Double Leader Treatment, spaced at 18”
3.     Four Leader Treatment, spaced at 18”

The results were surprising:  Single Leader actually resulted in the least labour over the season, in part because of efficiencies gained during harvest.  The Single Leader allowed for more cherry tomato grams harvested per minute.  The Four Leader treatment actually resulted in the most labour, as picking was slowed by the tangled growth, almost twice the amount of labour (training and harvesting) went into Four Leader production compared to Single Leader production.

Yields were comparable between the Four Leader and the Double Leader treatment, with the Single Leader treatment slightly lower.  However, Single Leader production resulted in higher yields earlier in the season when prices are the highest. 

If labour efficiency is paramount on your farm, or if you want to capitalized on early season prices, you might want to consider Single Leader pruning in your cherry tomatoes.  If you are looking for a balance between yield and labour, consider choosing Double Leader option.  You can read the full 2015-2016 Northern NY Agricultural Development Program Report here, and also take a closer look at the data here.