watering start and stop times

[keepleft]

the grodan website has some very technical information on water management guidelines.

 

one thing im curious about is this:

 

http://www.grodan.com/sw55949.asp

 

"Start and stop time

 

Irrigation start time and stop time, in combination with irrigation volume, can greatly influence the course of the water content in the slab and in turn, plant growth (Table 9.).

 

Early start time = 0-1hrs after sunrise (ie lights on)

Standard start time = 1-2hrs after sunrise

Late start time = 2-4hrs

 

Early stop time = 3-6 hrs before sunset (ie lights off)

Standard = 1-3 hrs before sunset

Late = 0-1 hrs before sunset

 

To illustrate these concepts, here are some examples of strategies to change slab Water Content (WC)

 

a) A late start (ie 2-4hrs after lights on) plus an early stop (ie 3-6hrs before sunset) in combination with a large irrigation volume per session (400-500 cc per m2) will cause the WC in the slab to gradually reduce over time. This is a generative strategy and would generally be adopted for winter and spring growing conditions in a northern climate with a winter planting programme. Under this regime, large Day / Night WC differences will be the result (hence the generative action). What’s more, Grodan®’s research is indicating less blossom-end rot develops when this strategy is adopted in the early part of the crop culture.

 

An early start (0-1hrs after lights on) plus a late stop (0-1hrs before sunset) in combination with small irrigation volumes per session (200-300 cc per m2) will result in continuously high WC in the slab and give the plants a strong vegetative influence. This strategy may be adopted during the summer production period in a northern climate zone in order to give the plants a strong vegetative influence. Be aware that early irrigation starts under low light conditions may cause unwanted root pressure leading to cracked fruit and soft, disease-prone plants. Botrytis and Mycosphaerella are encouraged under these conditions.

 

 

Irrigation volume per session

 

The applied irrigation volume (cc/m2) has less influence on the increase or decrease of the slab WC than the daily irrigation start and stop times. This does not mean that irrigation volume per session cannot be a useful tool for controlling WC in the slab. The usefulness of irrigation volume per session becomes evident for the effective control of EC and the stimulation of root growth and penetration (large volumes applied infrequently will stimulate root growth). Table 10 illustrates typical irrigation volumes defining small, medium and large applications per session.

 

Small - 150-300 cc/m2

Medium - 300 - 400cc/m2

Large - 400 - 500cc/m2

 

Irrigation frequency

 

Irrigation frequency or the number of irrigation sessions per hour will influence the WC in the slab. Many irrigation sessions per hour (>5 irrigations per hour) with a small volume will increase the WC in the slab and provide the crop with a vegetative influence. This strategy may be adopted during summer conditions in a northern climate.

 

Few irrigation sessions per hour (<1 – 1 irrigations per hour) coupled with a large volume per session will tend to decrease the WC in the slab over a period of several days. This approach may be used during the winter period in a northern climate zone in order to influence the crop in a generative direction.

 

Dripper output (cc/min)

 

Dripper output volume (cc/min.) will influence the WC rise in the slab during P1. The faster the water is delivered to the slab, the more “difficult” it is for the pores of the slab to retain and properly distribute the water throughout the slab. The result of using a high dripper output volume (60-70 cc/min.) is that the slab will begin to drain sooner and the WC during P1 will not be able to rise as high as compared to the application of water at a rate of 35–50 cc/min.

[Roadblock]

hi

 

It all sounds great on paper I wonder if the plant is supposed to know thats it should be this complicated , truth is the plant dont know it and it aint that complicated !

 

A plant as all things in nature has varibles to all its life processes , no varible is of such a fine degree , a plant with multible things being a little out multiplys the effects of bad but thats a whole new article on its own , all varibles concerned with plant growth have a scale of not good then a plateau of good and then a drop off to no good again , as a grower your job is maintain all the processes roughly in the middle of that good plateau .

 

I could give a drawn out response as to why but basicly you use as I said to you before, flow , Rockwool is about flow for one reason , Salt Buildup ! if you use short cycles with Rockwool you will have Salt Buildup within the slab for sure over time , monitor the salt by taking readings from your slab runout not the reservoir which you should be doing anyway as you want to know the readings arounds the roots not whats in res tank , feed enough flow so the salt reading coming out of your slab is the same as whats going in and you will not have a problem and the plants will be as good as any feed regime you want to quote , as the plants get bigger they place more demand on your system especially the root zone and salt can go through the roof very quickly in the guts of a Rookwool slab thats basicly been trickle feed , then you get a big kick in the ass that you didnt need for splitting hairs which really give you no benefit anyway .

 

Rod

[Luke Skywalker]

yeah, rockwool slabs are renowned for difficult management with salt levels, even though theyclaim it has no CEC.

 

Bottom line, your better off with a short irrigations more often whilst ensuring a decent volume of runoff to help with salt levels, and even then you're likely to end up with a need to flush regularly.

 

On top of all this, that piece by grodan is pointed toward tomato and not cannabis production. Tomatos have very, very different characters to MJ, and what works for them may well not work for MJ at all.

[Luke Skywalker]

And definitely check EC levels in the slab, it's the only real way to know. I've had this problem from experience, with EC of 3.6+ after very short times.

[thc24]

i think they start of ok but if you feed alot without flushing your in for trouble , always with RW check ec before watering because it only takes a missed weely flush to go toxic and once damage is done you will befighting it all the way, It seem the smaller the cube less the issue. water often like 6 to 8 times a day of small feeds about all it takes to wet not soak rw when young and increase when there is little to no run of try keeping rw on a bed of perlite ,ex clay this will help with drainage and cxan prevent most overwatering issues.

[keepleft]

hi again rod. can you tell me your irrigation volume (per slab) and frequency. both in veg and in flower. thanks in anticipation!!!

 

i used to feed for 15 mins every 4hrs (lights on only) and flow was huge (22.5lts per slab). everything went fine for yearssss (not 30 but many nonethless then pythium made me start thinking about water content and oxygen and letting ur medium dry out slightly b.w waterings

 

i got myself a digital timer and experimented with watering. when i watered too little, calcium deficiency showed up first. but reducing the watering definitely helped my pyth problem and root development

 

after trying a few permutations and combinations i spoke to my mate at holland forge who said 800ml per 2 hrs MAX in the slab for each crop box. im currently feeding 750 mls per slab every 2 hrs in flower and 1lt every 3hrs in veg. does that sound ok to you? flow is high (these feed times are 1 minute!) and run off good

 

my ec in the tank is 20 and ph 5.8. i take a reading of the EC in the medium every time (syringe 1inch into slab). the EC in the slab is 22 to 24 and ph is 5.9 to 6.2. sounds about right to me?

 

but im happy to change it if you can tell me whats the ideal water volume!

 

thanks

 

and luke - re tomatoes versus mj - i would have thought that water content issues would be the same? perhaps nutrient differences but wouldnt the same issues arise with water content irrespective of mj vs tomato??

[Roadblock]

I carnt tell you like that because it depends on the size and state of the plants the ambient temps humidity and the system , your room is dynamic and different to mine , different environment and different plants what is good for me might be too dry or wet for yours , I can see when my plants need feeding and I only need to be close to that time to be in cruise control .

For instance I have a small greenhouse clear UV covered , this time of year the greenhouse averages 45C in the heat of the day , I have heaps of airflow , I am feeding for 15 minutes every hr , with nutrient about 15CF and stay within 5.2 and 6.8 ph , they are powering , they will handle extreme heat if you have enough airflow , Im in Rockwool pots not slabs for this grow , but its the same principles anyway .

Another 2 mths from now and that will change to indoors with feeds about every 4 hrs for 15 min with CF around 25 , as you can see its horses for courses it really doesnt have to be complicated just be diligent in what you do and dont get slack , slack will always burn your ass just stay in the middle and have your basics covered .

 

Cheers

Rod

[Luke Skywalker]

Nah, water management is very different as tomatoes are watery fruit which have very specific needs, as well as being a totally different genera and family of plants requiring very different growing needs...

 

You sound like you're doing things about right, and in the end there is no "ideal" water volume. Every room is different, every plant is different, and everyone has their own way of producing the best. If you're having trouble with pythium or other water borne diseases you should look into more effective sterilisation or biological colonisation to offset this, but you sound like you may have already done this.

 

I'd always recommend under-doing it than overdoing it in rockwool as it still holds plenty of available water for the plants even when it seems fairly dry. It's just a matter of time and practice.

 

Hope that helps, and good luck.

[keepleft]

I am feeding for 15 minutes every hr , with nutrient about 15CF

 

2 mths from now and that will change to indoors with feeds about every 4 hrs for 15 min with CF around 25

 

hi rod im reading one of your old posts and saving them cause i like ur advice

 

anyway. can you give me more details as to why your using 15CF when u feed at 15 mins per hr, but 25CF when u feed at 15min per 4hrs???? thats a big difference in CF? is that because you have less runoff at the lower CF?

 

im feeding every 3hrs at 20 - 22CF.

 

also, do u find that when u feed a larger volume with less frequency you get a more developed tap root structure? at increased frequency and smaller volume i find the taproots dont develop and the roots just spread through the slab

 

final question - can u estimate the volume (in litres) per irrigation session (mines about 5lts - there 6 weeks into veg)

 

thanks mate

KL

Edited March 3, 2006 by keepleft

[Roadblock]

Hi

 

Keepleft its because of the drastic change in temps and humidity , in the heat they use heaps more water so your CF can skyrocket quickly ,, by using lots of low cf feeds I dont have to worry about the CF getting too high in the root zone , in winter the temps are cooler so their water intake slows so now they can take a higher CF without burning .

Volume I carnt tell you as Ive never measured it but its quite abit probably in the vacinity of 150 ltrs in 15 min total divide by 10 drippers , as I have decent size pumps .

 

If you have excellant drainage it doesnt really matter if you run 20 liters through them or 200 ltrs , the volume only has to flush the slab so the salt dont build up , thats the sole reason for high volume in rockwool .

 

The only tricky part is when they are young as Rockwool can get too saturated and oxygen starve the roots .

 

I also feel with CF you are better being a bit on the low side , the plant takes what it needs and because your cf is high doesnt mean they are assimulating more nutes , balance is more important than strength of the solution .

 

Rod