Best Nutrients For Autoflowering Cannabis

Autoflowers are becoming a popular choice of seed for many growers due to their ease and fast turnover. However, their small size means the nutrient requirements differ slightly from conventional (photoperiodic) plants. If you've been wondering what the best way to feed autoflowers is, you've come to the right place. Read on to find out more.

Why Autoflowers?

Autoflowers are great for a number of reasons but how their nutrients differ is another question. Because of their short life cycle and size, autoflowers do not require heavy amounts of resources between seed and harvest.

Growers can save a ton of money by not having to provide light or feed their plants for as long to achieve a result.

Other than their fast maturation, autoflowers have a strong, resistant nature and tend to require less nutrients because of it. They aren't overly fussy about the quality of the soil they are grown in and can still perform well with minimal nutrients. Depending on how they are grown, autoflowers generally reach flowering without the addition of external nutrients.

5 Reasons To Grow Autoflowers:

• Quick cycles (usually under 11 weeks from seed)
• They require less nutrients
• Strong, compact plants
• They do not rely on light schedules to know when to flower
• High mold and pest resistance
• Easy for beginners

Nutrients: Auto vs Photo 

There are some nutrient products on the market specifically designed for autoflowering cannabis plants but these are not necessary to grow autoflowers successfully. Bottled/liquid nutrients for soil or hydroponics work for autoflowers just as they do for photoperiods. The amounts we give the plants are just different.

Rarely do you need to add store bought nutrients at full strength and most brands suggest overly high doses. Either way, for an autoflower you will need much less than you would for a photoperiod strain. 

It's worth noting that the optimal pH for autoflowering plants is the same as for photoperiodic strains. In soil that would be around pH 6.5, and for hydroponics 5.8. Remember, the right pH for best nutrient absorption varies and depends on the stage of growth as well as the type of cannabis you're growing. 

Tip: Normally, a strain needs to be cultivated a few times before fully understanding how it grows with certain nutrients.

Best Nutrients For Autoflowering Cannabis

If you come from growing photoperiodic plants, then the feeding requirements of a cannabis plant will be familiar to you. However, if you're new to cultivation in general, then it helps to understand the exact demands of your plants so you can make sure your feeding them the correct amounts.

Other than water and air (hydrogen, carbon and oxygen), you mainly need to focus on nitrogen (N), phosphorous (P) and potassium (K) as the primary macronutrients. Other important elements include calcium, magnesium and sulphur as well as small traces of micronutrients, which are normally included as part of the line of nutrients you use.

The NPK ratio is labelled on most fertilizers. Which fertilizer line you use is up to you but choose a high quality range like General Hydroponics Flora, Fox Farm, or Advanced Nutrients. Experiment to see what works for the strains you're growing.

Be sure to dilute the nutrients down when you begin feeding autoflowers to avoid burning and stunting the plants. Photoperiodic plants aren't as dependant on time like autoflowers are, so less is better in most cases.

Nutrient burn during their cycle may result in an overall decrease in yields, so take care when you begin feeding autoflowers and preferably start with diluted strengths (1/4 or 1/2 the stated amount) to get a feel for how much the plants can handle.

Which Grow Medium Are You Using?

When and how much you feed your autoflowers depends a lot on the grow medium you are using. Soil tends to provide some nutrients whereas a hydroponics system does not. The way the nutrients are delivered to the plant is also different because in hydroponics there is not medium for the minerals to attach themselves to before being absorbed by the plant. 

Soil - Most soils for growing cannabis contain the right micronutrients and macronutrients to give autoflowers enough food to sustain them until the end of vegetation. If you notice yellow leaves before flowering start feeding but with a light dose, working your way up with each feed if the plants are handling it well. 

Hydroponics - In a hydroponics system, it is recommended to fill the reservoir with a weak nutrient solution once plants have 3-4 sets of leaves, unless they need it beforehand. Watch out for any signs of deficiency so you can be ready to feed from the moment the plants ask for it.

Coco Coir - Coco coir is an inert growing medium that provides very little nutrients to a cannabis plant on its own, meaning feeding schedules may need to be started earlier. Although there are nutrients for coco coir, hydroponics nutrients can be used instead without problems. Works very well when mixed with perlite.

Organic Super Soil - Overfeeding can be avoided by growing in organic soil made at home. This removes the need to add any external nutrients at all, unless the plants demand it. Natural ingredients are broken down more slowly meaning it is not as easy to overfeed plants this way.

Root Supplements

In order to stimulate root growth and health, we can add root boosters to our soil or watering solution. Although not essential, these bacterial supplements can help to break down organic matter into food for the plant. In addition, the bacteria provide protection to the roots and encourage their development. Many growers use these throughout the grow cycle to avoid root problems.

Feeding Autoflowering Cannabis

It's easy to fret over NPK ratios but most autoflowers tend to grow well as long as they have some of each element available to them. Cannabis plants in vegetation generally needs plenty of nitrogen, increased phosphorous during preflower which continues with higher potassium for the flowering stage until ripening. The nutrients are then reduced signficantly in the last days of flowering.

Feeding is easy when it comes to autoflowers but the challenge is not to overfeed. Heavily fertilising an auto is hard to go back on. Considering that autoflowering varieties mature based on their age, any heavy stress they face can massively slow them down if not managed correctly.

Seedling

A cannabis plant is very fragile at this stage of growth due to its underdeveloped root system. It does not yet have the capacity to absorb heavy nutrients and relies on nutrients stored in the seed and present through the atmosphere. Only if a seedling displays signs of deficiency should you start feeding. If that is the case, dilute at a very low strength and feed with every other watering.

Recommended NPK ratio: 2:1:2

Vegetative

Autoflowering cannabis plants have an almost non-existent vegetative phase and basically jump straight from seedling to flowering. To get the most growth out of your plants, it may be a good idea to start adding more nitrogen into their diet so that plants develop strong branches and leaves before the pre-flowering stretch. A healthy autoflower begins vegetation at about 10 days from seed, or once the first 5 finger leaves develop.

It is best to gradually shift nutrient schedules so that we do not increase the risk of shocking or stunting our plants. As we approach the blooming phase, we can even mix our grow and bloom nutrients to create a balanced mixed while the plant makes the shift.

Recommended NPK ratio: 10:5:5

Pre-Flowering

The preflowering stage is usually the period of explosive growth as a cannabis plant begins to flower. For autoflowers it's a bit different because they do not mind as much about a sudden shortage of light. The stretch for autoflowers lasts about 1-2 weeks and isn't nearly as heavy as it is for photoperiods (which happens mainly because of the drastic change in light cycle).

Some autoflower growers will stick with vegetation nutrients for the first weeks of flowering. The continued addition of nitrogen during the early blooming phase can encourage growth but too much may mean the plant ends up growing too many leaves. Keep an eye out for early yellowing, which may mean you need to up the dosage. An increase in phosphorous during this phase can also help to kickstart flower production.

Recommended NPK ratio: 10:10:5

Flowering

As an autoflower develops into full bloom, nitrogen can be reduced. The schedule is switched to give higher levels of phosphorous and potassium. If anything, give autoflowers plenty of phosphorous as it's necessary for bud production, or they may deliver underwhelming harvests.

PK 13-14 is a great product for adding supplementary P and K without N. However, nitrogen is still important to some degree so make sure the feeding solution also contains some. As the plant finishes its 'stretch', consider further decreasing nitrogen content and upping the phosphorous and potassium until late flowering.

Sweet Seeds Black Jack Auto

Depending on how heavily your feed an autoflower, you may want to start reducing the given nutrients earlier or later. Consider giving the buds a day or two to mature without feeding right up until harvest day.

Recommended NPK ratio: 5:15:10

Feeding Schedule For Autoflowers

After all that, it might be useful to lay out an example for a typical feeding schedule for an autoflowering cannabis plant grown in a soil, coco and perlite mix. Bear in mind that the strains you're growing may perform better (or worse) with other diets. 

Some autoflowers can handle a heavier nutrient schedule which could indeed lead to larger yields, so experiment to find out what works best.

Autoflowering Nutrient Tips

Buy high quality nutrients - The higher-quality nutrients you purchase, the better. Choose reputable products that have a history of success and don't be fooled by feeding schedules. Nutrient manufacturers often overstate the dosages of their fertilizers.

Calcium & Magnesium - Cal-Mag supplements are recommended if you are not growing in soil. Calcium and magnesium are both important secondary macronutrients essential for healthy growth but they are easily washed away in soilless grow mediums. They can be added alongside your feeding schedule and reduced throughout flowering.

Mix one nutrient at a time - When preparing your nutrient solution or adding supplements, add each part separately so that the minerals dissolve evenly into the water. 

Check pH of nutrient solution - After adding nutrients to water, let the solution sit for 10-15 minutes and check the pH. Adding fertilizer to water tends to bring down the pH, turning it more acidic. Even if you use the best nutrients on the market, if your pH is out of the proper range your plants will not be able to use them. Plants benefit from a varied pH between feedings as not all minerals are absorbed in the same range.

Use an EC meter - An EC/TDS meter can help you determine whether your nutrient solution is too strong for your autoflowers. Most nutrients come with a feeding chart which indicate PPM (parts per million), or the concentration of minerals and salts present. Again, this depends on the strains your growing as well as the stage of growth.

Choose nutrients suited to your grow medium - Natural fertilizers are well suited for soil grows due to their slow release, as well as the lack of residue they leave behind. Synthetic/liquid fertilizer is best for hydroponics systems and other soilless mediums because nutrients in this form become directly available to plants where they do not have a substrate to attach themselves to.  

Give water at room temperature - Avoid shocking your plants by making sure the feeding solution is at 22-23°C before watering. PH is also affected when the temperature fluctuates out of the comfortable range.

Flush in the event of nutrient burn - Autoflowers tend to be sensitive to over-feeding but a quick flush can help to remove any potential salt build up around the roots. Some growers get into the habit of flushing once or twice during the grow cycle, regardless of whether there is a problem with nutrients. A thorough flush may not be necessary, but giving water every so often between feeding can help to avoid nutrient burn.

Autoflowering grow by QuantumSkies from GrowDiaries.

Conclusion

The best nutrients for your autoflowers comes down to many factors, as you have seen here. Covering the basics such as lighting, pots and ventilation should be your first step, but do not forget the importance of fertilizers if you're looking to take your grow operation to the next level.

We'd love to hear your thoughts on the best nutrients for growing autoflowers and the schedules that worked for you. Share your experience in the comments section down below!

External References

Impact of N, P, K, and Humic Acid Supplementation on the Chemical Profile of Medical Cannabis (Cannabis sativa L). Frontiers in Plant Science. - Nirit, Bernstein & Gorelick, Jonathan & Zerahia, Roei & Koch, Sraya. (2019).

Hydroponics Cultivation Cannabis sativa L. Plants. Planta Medica - PLANTA MED. - Chandra, Suman & Lata, Hemant & Khan, Imtaz & Elsohly, Mahmoud. (2010).

Liquid organic fertilizer production for growing vegetables under hydroponic condition. International Journal of Recycling of Organic Waste in Agriculture. - Phibunwatthanawong, Thanaporn & Riddech, Nuntavun. (2019).

Effect of Irrigation and Fertilization Levels on Mineral Composition of Cannabis sativa L. Leaves. Notulae Botanicae Horti Agrobotanici Cluj-Napoca. - Wogiatzi, Eleni & Gougoulias, Nikolaos & Giannoulis, Kyriakos & Kamvoukou, Christina-Anna. (2019).

Physiological and biochemical features of Cannabis ruderalis in the Chui valley. Journal of Biotechnology. - Akhtaeva, Nursulu & Boribay, Elmira & Mamurova, Assem & Kiekbaeva, Lashyn & Inerbaeva, Saniya. (2014).

Isolation and investigation of antibacterial properties of preparations from wild hemp (Cannabis ruderalis) growing in the Ukraine. Mikrobiolohichnyĭ zhurnal. - RABINOVICH, A & AIZENMAN, B & ZELEPUKHA, S. (1959). 

This article was updated November 2020.