High Solubility Salts and Fungus Gnats in a Geranium Crop Ten Days from Ship
You are finishing 6-in geraniums in a 40 × 96 ft greenhouse. The crop was potted five weeks ago into a peat/perlite mix with a wetting agent and no incorporated lime. You fertigate at every irrigation with 20-10-20 at 2 lb per gallon of stock through a 1:100 injector. Today's pour-through test reads pH 5.1 and EC 3.2 mS/cm. The geraniums show lower-leaf yellowing, marginal necrosis on older leaves, and some leaf drop. Day/night setpoints are 68/62°F; roof vents have been running from about 10 a.m. daily and afternoon RH drops to 35%. Yellow sticky cards near the crop are catching fungus gnats, and you find larvae in the top 1/2 in of media in several pots. The crop ships in 10 days for a Memorial Day sale, and the buyer rejects plants with yellow lower leaves or visible insects.
1. Check the injector math and interpret the pour-through numbers. What is the most likely cause of the symptoms, and what corrective action fits a 10-day window?
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Math first. Two pounds of 20-10-20 per gallon of stock contains 0.4 lb of actual N, about 181,000 mg. In 3.785 L of stock that is roughly 48,000 ppm N, and at a 1:100 injector the delivered concentration is about 480 ppm N - two to three times the 150–200 ppm N used for continuous feed. That alone explains an EC of 3.2, well above the roughly 1.2–1.8 mS/cm you would expect for a 6-in geranium pour-through.
pH. 5.1 is below the 5.8–6.2 range. 20-10-20 is a strongly acidifying formula, and the mix had no lime to buffer it, so five weeks of feeding has pulled pH down. Low pH plus high salts produces exactly this pattern: lower-leaf yellowing, marginal necrosis, and leaf drop from salt injury rather than a micronutrient deficiency.
Action. Leach each pot with clear water at about two to three times the container volume until pour-through EC falls under 1.5. Then switch to a less acidifying, higher-nitrate formula (13-2-13 or 15-5-15) at 150 ppm N. Correct pH toward 5.8 with a flowable lime drench, but do not overshoot right before shipping - a slow drift back is safer than a panic drench. Verify the injector with a draw test; a 1:100 injector that is actually pulling 1:50 doubles everything above.
2. Design a fungus gnat program that can be finished before the ship date without leaving residues or visible adults.
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The larvae in the top half-inch plus adults on cards confirm an active breeding cycle, not a stray. Gnats need wet media surface, algae, and organic debris, so cultural control comes first: stretch irrigation intervals enough to let the surface dry between waterings, increase airflow, fix hose leaks and algae under benches, and remove weeds and spilled media.
The 10-day clock rules out a single application. Biological control is the right fit: Bti (Bacillus thuringiensis israelensis) drenches kill larvae and are applied at 5–7 day intervals, so you get two, possibly three, treatments before shipping. Steinernema feltiae applied to the media surface works well in combination and is compatible with Bti. If pressure is high, an insect growth regulator such as diflubenzuron can be added for longer residual and is compatible with beneficials.
Avoid pyrethrin or other adulticide aerosols late in the crop: they knock down flyers briefly, leave residues on foliage, and select for resistance. Use yellow cards to monitor rather than to control.
Set a shipping standard now: no adults on cards for 48 hours and no larvae in a random sample of ten pots. If adults are still emerging at day seven, hold that block and tell the buyer rather than shipping live insects into a retail bench.
3. The grower wants to raise the night temperature to push the crop and improve color before the sale. Evaluate that idea against the data in the scenario.
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It treats the symptom the buyer sees, not the cause, and it costs money at exactly the wrong time. The crop is already salt-stressed with root function compromised; pushing growth with more heat increases transpiration and nutrient demand from a damaged root system, which usually deepens marginal necrosis rather than greening the plants.
There is also a quality argument. Raising night temperature reduces the day/night difference and promotes stretch, so you get taller, softer plants with poorer branching right before sale - the opposite of what a Memorial Day bench wants. Geraniums color best with high light and moderate temperatures; if the crop is pale, the first questions are pH, EC, and root health, not thermostat setpoints.
A better pre-ship plan: correct the salts and pH, hold 68/62°F, maximize light and airflow, and do a light clear-water leaching if EC creeps back. If the grower needs faster finish, a modest increase in day temperature with strong ventilation is safer than raising nights, and it should only be considered after the leach and formula change have had a week to work.
Document the recommendation in writing. If the grower overrides it, the record protects you when the buyer rejects yellow lower leaves.