Feeding, Pot Size and Flowering Onset in Cannabis

Imagine two plants of the same variety that differ only in how they are fed, and ask whether the heavily fed one flowers later. A growing operation describes flowering onset shifting by up to several weeks under nitrogen-heavy feeding, but this delay has not yet been measured in cannabis. The opposite direction has been measured: without nitrogen fertiliser, field hemp reached full flower later.

Does too much nitrogen delay flowering?

This has not been measured in cannabis, but it has in a model plant, thale cress, known botanically as Arabidopsis. Its flowering time follows the nitrate supply along a U-shaped curve. Nitrate is one of the two forms in which roots take up nitrogen, a macronutrient that plants need in large amounts. Plants with a medium supply flower earliest, and both more and less nitrate delay flowering. Delay from excess has been described in plant research since 1913.

The low point of the curve sat in a different place in each growing system. Besides soil, the table includes hydroponics, meaning growing without soil in a nutrient solution of water and dissolved fertiliser, and agar, a firm gel for lab plants. The amounts are given in millimoles per litre, a unit of quantity used in chemistry.

For cannabis there is only the practitioner report: under nitrogen-heavy feeding, flowering onset came up to several weeks later. The leaf tissue was dark and soft, and some plants reverted to vegetative growth.

Thale cress growing system	Nitrate	Days to flowering
Soil	4 mmol/L	28.6, earliest
Soil	1 and 10 mmol/L	about four days later each
Hydroponics	1 mmol/L	33.3, earliest
Hydroponics	2 mmol/L	40.1
Hydroponics	0.1 mmol/L	45.1
Agar	1 mmol/L	21
Agar	35 mmol/L	31

Do not transfer any thale cress nitrate value to cannabis.
To test it yourself, split clones, meaning rooted cuttings, from the same mother plant into two feeding groups.
In both groups, note the date of the first flower set at the shoot tip.
Look up the nitrogen amount before and during flowering and the tolerable ammonium share in the article on nitrogen.

Does a hungry plant flower earlier?

No, in the field the hungry plants flowered later. With 90 kg nitrogen per hectare, three hemp varieties in northern Nevada reached full flower 17 days earlier on average than without extra fertiliser. Depending on the variety, the difference was 6, 13 or 21 days. This does not contradict the U-shaped curve, because the trial only compared deficiency with adequate supply. That covers just the left arm of the curve.

The poorly supplied plants showed single early flowers in the leaf axils, the angles between leaf stalk and stem. At the shoot tips, however, these plants flowered later, and flowering across the stand stretched over 15 days. Anyone who takes the first axil flowers for flowering onset therefore sees the starving plant ahead. In a veg pot trial, the number of leaf pairs on the main stem was the same between 30 and 320 mg/L nitrogen. Nitrogen thus changed the size of the plant, but its pace of development stayed the same. A small plant looks finished earlier because less mass has to ripen.

Read flowering onset at the shoot tip and do not count single flowers in the leaf axils.
Do not use starving as a flowering trick.
Do not count a small plant that looks finished early as an early switch to flower.

Does the flowering curve also apply to cannabis?

This has not been tested in cannabis. In thale cress, nitrate affects flowering time more strongly under short days than under long days. In one paper it flowered earlier at scarce nitrate only under 12 h and 8 h of light, and not under 16 h. The hemp varieties tested flowered as soon as the day fell below a variety-specific length, although some fibre and grain varieties also flowered under 18 h. The U-curve has also been described in a duckweed that likewise flowers on short days. Thale cress itself, by contrast, flowers in response to long days, and a test across many species is still pending.

In thale cress, nitrate and phosphate act in opposite directions, phosphate being the form in which the root takes up phosphorus. Less nitrate promotes flowering, and so does more phosphate. A lot of nitrate also lowers the phosphate content of the plant, whereas ammonium, the second nitrogen form, does not. The gene FLC slows flowering and follows nitrate, while the gene FT triggers it and follows phosphate. Less nitrate than in excess thus brings flowering forward by two routes.

Do not plan on phosphate as a flowering trigger while this route is unmeasured in cannabis.
Set the phosphorus amount by need and look it up in the article on phosphorus.
Record your own flowering onset in every run with a date, so you can measure a fertiliser change against it.

Does a small pot bring flowering forward?

This has not been measured for cannabis. Root restriction is a named growing technique that deliberately keeps the root zone small. It is in commercial use in Chinese viticulture and has been described in five other fruit and vegetable species, including apple and tomato. Its stated purpose is to curb shoot growth and to promote flowers and fruit. In grapevines, available nitrogen in the root zone at full bloom, the point when most flowers are open, was 187 instead of 859 mg/kg soil. At ripening it was 130 instead of 258 mg/kg. Phosphate fell less, to 220 instead of 402 mg/kg at full bloom, and at ripening it was even higher under restriction. According to earlier work by the same group, abscisic acid, a hormone that plants form under stress, rises in the process.

A small root zone thus shifts the ratio towards phosphate, which in thale cress is the direction of early flowering. Both links of this chain are measured, but in two different species. A growing operation reports that long-flowering tropical varieties only ripen in time in a restricted pot. According to the report, stretch and ripening can be steered this way. That may be an earlier switch to flowering or a smaller plant that finishes faster, and a measurement separating the two is still missing. In other species a tight pot has been shown to keep the plant smaller, but no paper read here measures an earlier switch.

Do not plan on a small pot as a flowering switch.
With tropical varieties in a restricted pot, record two dates separately: the first flower at the shoot tip and harvest maturity.
For comparison, grow clones from the same mother plant in two pot sizes.

How big does the pot need to be?

What matters is the ratio of plant mass to root volume, which specialists call the biomass-to-volume ratio. They measure it in grams of dry mass, the weight of the plant without water, per litre of substrate, the material in the pot such as soil or coco. Below 2 g/L pot effects stayed small, and above it they became marked. Across 65 trials on 69 plant species, doubling the pot gave 43 % more mass on average, but cannabis was not among them. Above all, a tight pot lowers net photosynthesis, the sugar the leaves gain after subtracting their own respiration. This brake also works in hydroponics with full supply.

The value of 1 g/L is a recommendation for experimental setups and is not a growing guideline, since its purpose is to keep a pot effect from distorting an experiment. The median of real pot trials, meaning the middle value of all of them, was 9.5 g/L. For field crops, estimates come to 1 to 3 g/L. If you took 1 g/L as a garden rule, a plant with 500 g dry mass would need a 500 l container. The ratio rises by itself, because the plant grows and the pot does not. Small pots also heat up more: 0.19 L pots were 5 °C warmer by day than 1.9 L pots.

Divide the expected dry mass by the pot volume. For example, 100 g by 11 l gives about 9 g/L by calculation.
Do not read 1 g/L as a growing guideline.
Repot in time, because a pot that is roomy for a seedling can cramp the same plant eight weeks later.
Protect small pots from direct sun and heater warmth.