Feeding and Root Volume: Their Influence on Flowering Onset, Duration and Plant Size

Two plants of the same variety, same age, same spot, only fed differently. The one on heavy nitrogen is tall, dark green, soft-leaved — and it starts flowering weeks after the other one, sometimes even reverting to vegetative growth. This is not growing lore: flowering time follows nitrogen supply along a curve with a low point in the middle. A moderate supply flowers earliest, more delays it, and less delays it too. The arm of the curve where excess causes delay has been described in plant research for over a hundred years.

How much nitrogen shifts the date

Nitrogen is the nutrient a fertiliser delivers in the largest quantity and the one that makes leaves green. The most precise figures on flowering date come from a small model plant where such things can be tested cleanly. In soil it flowered at moderate supply after 28.6 days; at more than double that amount and at a quarter of it the date sat roughly four days later in each case. In nutrient solution without soil the gap was wider: 33.3 days at the low point against 45.1 days at a tenth of that amount, close to twelve days apart.

The strongest shift went upward. In a third setup the same plant needed 31 days to flower at 35 parts nitrate and only 21 days at one part — ten days from fertiliser level alone, without anything changed in the light.

Where exactly the low point sits depends on the growing system. In soil it lay three to four times higher than in nutrient solution, because availability fluctuates in a substrate, declines in a gel and stays constant in a circulated solution. Transferring a figure from one system to another, the authors warn explicitly, is not valid, and that applies all the more to the jump from the model plant to cannabis.

Why phosphorus works in the other direction

Nitrate and phosphate are the forms in which the root takes up nitrogen and phosphorus, and they pull against each other where flowering is concerned. Less nitrate promotes it, more phosphate does too. The antagonism starts at the root: a high nitrate supply lowers phosphate accumulation inside the plant, and only nitrate does this — ammonium, the other nitrogen form, has no such effect.

Inside the plant the two act at different points. The gene that delays flowering tracks the nitrate supply; the gene that triggers it tracks the phosphate. Because the phosphate content of a plant correlates with its level of flowering-promoting hormones, the phosphorus route may run through that pathway. Early flowering under scarce nitrogen therefore has two causes at once: the lower nitrate itself and the higher phosphate content it produces.

What transfers to cannabis

Two things support the transfer. The nitrate effect on flowering time is stronger under short and medium days than under long days; in one study early flowering under scarce nitrate appeared only at 12 h and at 8 h of light and not at 16 h. Cannabis flowers on short days. And the same U-shaped response has been described in two further species that also flower on short days.

Two things work against it. The model plant flowers on long days and therefore runs on different control machinery, and the authors note that only a systematic test across many species can show how general the curve is. A trial that pushes cannabis through a nitrogen range beyond the optimum while recording the flowering date does not exist.

What practitioner experience adds matches the direction: a grower observes that the same variety under nitrogen-heavy feeding delays its flowering onset by weeks, develops dark and soft leaf tissue, and sometimes reverts to vegetative growth, while phosphorus and potassium at the right moment carry the flower through.

Why a field trial shows the opposite

Three hemp varieties in northern Nevada reached full flower on average 17 days earlier when fertilised than when left unfertilised. This does not contradict the picture — it sits on the other arm of the curve: the unfertilised soil there still carried measurable nitrate and ammonium, and the addition of 90 kg per hectare moved the plants toward the low point rather than past it.

A second finding from the same trial explains a widespread misreading. The poorly supplied plants showed early flowers in the leaf axils and reached flowering at the shoot tips later; their transition stretched over a good two weeks. The individual flowers in a leaf axil form as the plant ages, even while nights are short. Taking them for the start of flowering makes the starving plant look like the more advanced one.

When nitrogen arrives as ammonium

For cannabis the form is decisive. At a constant nitrogen level with a rising ammonium share, flower mass fell against pure nitrate supply by 35 % at a tenth ammonium, by 46 % at half, and by 97 % when all nitrogen came as ammonium. The recommended ceiling is one third.

More important than the figures is the time course. Ammonium toxicity accumulates and shows late: at half ammonium the plants looked unremarkable through most of the trial, showed damage only in the last two weeks, and four in ten died. A liquid organic fertiliser typically carries a higher ammonium share than a mineral one; when both types were compared, the highest organic rate produced burnt leaf tips.

Feeding less once flowering is under way

Three fertiliser rates during flowering produced 16.1 g, 22.6 g and 26.3 g of flower dry mass per plant — roughly a third between the lowest and the highest. The active compound concentration ran in the opposite direction: 6.3 % at the leanest rate against 5.7 % at the richest. On balance the middle rate delivered 95 % of the compound yield of the highest, with a third less fertiliser.

Behind this sits remobilisation: the transfer of nutrients from older leaves into the flowers. Under lean supply it sets in earlier — in this trial roughly four weeks earlier than at full feeding — and it shows as lightening and later as red and purple colouring of the lower leaves.

What the pot changes

Root restriction is not a makeshift but a named growing technique. In Chinese vineyards it is in commercial use, and it has been described on nectarine, sweet cherry, chilli, apple and tomato. Its declared purpose is exactly what matters here: to curb vegetative growth and promote generative growth. Roots grow thinner and longer under restriction, shoot vigour drops markedly, and the distribution of assimilates shifts from shoots to fruit.

How strongly the intervention acts shows in the nutrient content of the root zone. In restricted grapevines the plant-available nitrogen at full bloom stood at 187 mg per kilogramme against 859 mg in unrestricted ones; at the ripening stage it was 130 against 258 mg — roughly a fifth to a half. Available phosphate dropped as well, but less sharply: 220 against 402 mg per kilogramme. The same group notes that root restriction limits nitrogen availability and finds elevated levels of abscisic acid, a hormone the plant forms under stress and one that drives ripening.

This closes the circle to the beginning of this article. A smaller pot lowers the nitrogen supply above all, and the phosphate less sharply — it shifts the ratio of the two in the same direction that brings flowering onset forward. For cannabis this chain has not been measured; its two links have, in different species. A grower reports that long-flowering tropical varieties can only be brought to harvest in time through a limited root volume, and that both stretching and ripening can be steered this way.

For the size effect there are figures from a review of 65 trials across 69 plant species, cannabis not among them: doubling the root volume raised biomass by an average of 43 %, mostly because the leaves in the tight pot produce less. The recommendation given there, that plant dry weight should not exceed 1 g per litre of substrate, applies to experimental setups and is not a growing guideline: it is meant to prevent a pot effect from distorting a trial. Real pot trials sat at a median of 9.5 g per litre, and for field crops the same authors estimate 1 to 3 g per litre. Anyone following that figure in a garden would need a 500-litre container for a plant with 500 g of dry mass.