Is Lowering Fertilizer Right For You?

This guest post was written by Jessie deHaan, Research Technician at Vineland Research and Innovation Centre (Vineland).

Four chrysanthemum varieties growing side by side at 75ppm (left side of benches) and 250ppm of N (right side of benches) of N in a Vineland trial.

Economical and high quality plant production has always been a balancing act between fertilizer, light, irrigation and other input costs.  

Unfortunately, supply chain issues caused by the pandemic slow-down, and exacerbated by the subsequent war in Ukraine, have negatively affected the global supply of fertilizer and plant nutrients. These supply issues are leading to rising fertilizer prices for growers.

As input costs rise, how can growers respond and save money, while still producing first-rate products?

Fertilizer and Floriculture:

Previous studies at the University of Guelph by Shelp, Zheng and MacDonald indicated that floricultural crops often receive more fertilizer than they need. Besides the obvious economic benefits (lower input costs), lowering fertilizer input can also prevent nutrient runoff and protect lakes and waterways from increased phosphorus and nitrogen levels escaping from nearby farms. But, is it possible to lower fertilizer input but still produce the same high quality plants? If so, how low is too low?

As part of a larger project, funded by Canadian Ornamental Horticulture Alliance (COHA) and American Floral Endowment (AFE), to study the effect of reducing fertilizer on plant susceptibility to thrips populations, Vineland and OMAFRA researchers conducted a series of experiments on potted chrysanthemum (mum) using different fertilizer levels.

In Trial 1, we grew Springdale purple mums in an older greenhouse (with less than ideal growing conditions) at five different levels of 17-5-17 fertilizer: 25, 50, 100, 200 and 300ppm of nitrogen (N). This was to understand just how low we could go while still producing sellable plants. In Trial 2, we grew four varieties of mums in a newer greenhouse with more modern sensors and environmental control (varieties: Springdale purple, Grandview pink, Vyron yellow, Chesapeake yellow). These mums were grown at two levels of 17-5-17 fertilizer: 75 and 250ppm of N. These rates represented more realistic low and high fertilizer rates, based on our results from Trial 1.

All plants were grown according to standard industry guidelines for pot mums. We measured various parameters such as time from transplant to bud crack, number of flower buds and plant height to see if we could reduce fertilizer costs without affecting the salability of plants.

Our Findings:

Figure 1. Comparing plant quality of potted chrysanthemum grown at 75ppm (left) and 250ppm of N (right) in Trial 2. From top to bottom: Springdale purple, Chesapeake yellow, Grandview pink, Vyron yellow. Note the extra foliar mass produced in Springdale at high fertilizer and the longer flower stems produced in Vyron at low fertilizer.

The good news is that when chrysanthemums were grown under ideal conditions (in the newer greenhouse), plants fertilized at 75ppm of N were indistinguishable from the higher fertilized plants (250ppm of N, Figure 1) in terms of bloom number and plant size .

This confirms that in newer operations, when lighting, temperature and irrigation are ideal, lowering fertilizer rate is a realistic option to save costs without sacrificing quality.

However, during our earlier trial, where ventilation and heat pockets were a problem due to older infrastructure, things didn’t go quite as expected. The lower fertilized plants did not perform as well (Figure 2). This suggests that fertilizer rates of 100ppm of N and below are limiting plant growth under these conditions.

We also found that not all varieties respond in a similar way to lowering fertilizer.

For example, Vyron had longer flower stems at lower fertilizer, while none of the other three varieties responded this way. Interestingly, Springdale produced top-heavy, floppier plants at high fertilizer.

Time from transplant to bud crack was another interesting finding (Table 1). Most varieties started to bloom during weeks eight and nine in Trial 2, which is approximately the industry standard for the time of year when they were grown.

However, Springdale took an additional five to seven days to show colour and this delay was even greater in the older greenhouse used in Trial 1. In that trial, some plants were two to three weeks later than the catalogue growth time.

Table 1. Metrics of plant growth for four chrysanthemum varieties grown at low (75ppm of N) and high (250ppm of N) fertilizer rates in Trial 2. Vyron plants grown at lower fertilizer had longer bud stems, leading to a taller looking plant. Plants grown at lower fertilizer (75ppm of N) had just as many flowers as plants grown at higher fertilizer (250ppm of N) in a modern greenhouse. Flowering of Springdale variety was delayed by about one week in both high and low fertilizer groups, likely due to factors other than fertilizer rate.

Chrysanthemum VarietyFertilizer Rate (ppm of N)
Plant Height (cm)Low (75ppm)High (250ppm)
Number of Flower Buds  
Transplant to Bud Crack (Weeks + Days)  
Chesapeake (catalogue: 8 + 4)8 + 48 + 4
Grandview (8)8 + 28 + 2
Springdale (7 + 4)8 + 4*8 + 2*
Vyron (9)99 + 2

This isn’t necessarily surprising given that heat delays flowering in chrysanthemum and Trial 1 was conducted during a hot summer in an older greenhouse with poorer ventilation and air circulation. Therefore, variety selection should be made with greenhouse conditions in mind or else you will need to account for these expected delays

The Importance of On-Farm Tests:

Because the effect of fertilizer reduction on potted chrysanthemum quality can be variety dependent and environmental conditions can vary between greenhouses, if you are interested in lowering fertilizer levels you will likely have to find the “goldilocks” rate for all varieties grown in your operation.

This might involve putting different compartments or benches on different feed tanks. Test a small cohort around 10 plants) of each variety in the same year and production cycle, in an area of your greenhouse typically used for production. Test on these cohorts several fertilizer rates between 100 and 300ppm of N to find the optimal one that works for your greenhouse’s growing conditions while still producing beautiful plants on schedule (Figure 2). If feasible, lowering your fertilizer by 100ppm of N could lead to a significant cost savings (see the “How Much Can I Save” section, below).

Figure 2. From left to right, the range of plant quality of Springdale purple potted mum grown in Trial 1’s less-than-ideal conditions at 25ppm, 50ppm, 100ppm, 200ppm and 300ppm of N. Plant quality suffers significantly below 100ppm. However, between 200 to 300ppm of N plants were fairly close in quality. This shows the importance of taking photos of every variety in trials testing out reduced fertilizer levels.

Lastly, take photos and document results for future reference. Examples of data that can be easily collected include: number of flowers/buds, plant height, production time and overall quality. Compare the plants produced at lower fertilizer rates to those produced in normal production to determine whether plant quality and production time are acceptable.

How Much Money Can I Save?

Depending on your growing conditions and varieties, you may see little to no difference in plant quality by reducing fertilizer input and potentially save money. For example, if a 15 kg bag of fertilizer costs $65, and you’re fertilizing at 250ppm of N using a 1% injector, you can make 10,200 L of feed solution. If every 6-inch pot in your greenhouse needs 13 L of feed over eight weeks, growing 10,000 pots will cost you $828 (at $0.083/pot). If you reduce your feed by 100 ppm to 150ppm of N, this same bag can make 16,900 L of solution. The same feed rate for the same number and size of pots would net you $300 in savings for each crop (costing only $0.05 /pot). (Numbers are based on calculations by Dr. Neil Mattson, Cornell University using current fertilizer prices).

Other work on the topic:

In previous research trials by Barry Shelp et al., it was found that sub-irrigated chrysanthemums grown under research or commercial greenhouse conditions showed no deficiency or quality issues when grown at 75% lower nitrogen than the industry recommended 200 to 300ppm of N. Final plant and flower quality remained consistent across a wide range of fertilizer rates.

Calcium, magnesium, zinc and copper could also be significantly reduced (75-85%) without observing a deficiency in flowers, leaves or shoot tips. However, the effect of reducing these elements is not consistent across mum varieties, meaning that the effects of lowered nutrients can be variety-dependent .

Take home message:

Our results are consistent with other trials and point to the exciting potential of saving money when it comes to producing potted mums on reduced fertilizer. And this can help the environment, too!

However, it’s important to remember that some crops or varieties may be more negatively impacted than others, so test the effect of reduced fertilizer on all crops and varieties being grown before making sweeping changes. Avoid making generalizations for all varieties if favourable results are found for one variety. Take the results of any single variety trial with a grain of salt – the only way to know the effect on your crop in your greenhouse is to test it yourself.

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