RH importance

Besides the pests and mildews/molds, why else is it important to have 60-80% RH in veg, and 30-50 RH in flower...? Does it have to do with the way the plant transpires in diff. phases..? I just dont understand why flowering phase HAS to be 50 or below..? Ive had my veg room at 40-60 RH, and my flower from 35(day) to 65(night) RH, and noticed no probs.

rh/vapor pressure

rh/vapor pressure

relative humidity is an expression of the actual water vapor pressure, expressed as a percentage of the maximum water vapor pressure possible under certain air+atmospheric pressure conditions.

@ room temp (~60*f), 100% humidity exerts a vapor pressure of 24 torr (~4.65 pound-force per sq "[24*{19.337*10^-3}]=4.65 psi pound-force). >24 torr of vapor pressure exerted on leaves, and leaves sense a vapor pressure deficit.

leaves stomata opening/closing influenced by difference between internal/external vapor pressure. opening/closing of stomata regulates gas exchange+transpiration, which in turn regulates growth/fruiting.

vapor pressure deficit is a lack of water pressure upon plant. this would be a low rh. it is indirect measure of water loss from plant. as plant attemps to balance internal/external vapor pressures, they draw up more water from roots and transpire it into the atmosphere. hence the de-humidifiers used in gardens.

air movement over plant+high temps+low rh reduce plants available water for sugar production. the roots uptake much more water during low rh. but all inter-related to ambient temp(avg. surrounding air temp), actual leaf temp, and root temp.

a 3'x3' plant can transpire a gal. during high vpd (low rh), or a cup during low vpd (high rh). vpd/rh measurements must take into account air temps.
generally, keeping rh as high as possible (~55-75%) keeps the plant in a comfort zone to its genetic desires. lowering rh provides a method of tailoring nute formulas, pushing parameters, etc - as they will be taking up much more water. keep in mind that the water uptake during low rh is mostly just that - water. keeping nute ppm/ec in balance w/ ph is good idea.

prevention of mold/diseases/condensation is the reason for decreasing rh, but it can be higher w/ air exchange/movement w/ tempered evaporation (air movement over/around plants). also, growth slows during high rh, but in fruiting, slow growth is not an issue, fruit production is. lower rh also increases translocation of calcium (why gardeners use calcium additives during fruiting - it is sent to every part of plant during increased water uptake via roots).

not having problems because the plants' comfort zone is pretty broad w/ comparable temps+air movement - just like there's a 'domain+range', so to speak, in nute levels, etc.

note: in general...
lower rh(high vpd)=increased transpiration, translocation, water uptake, greater calcium absorption/transport.

higher rh(low vpd)=slower transpiration, translocation, water uptake, slower evaporation, increased growth.

rh does not need to be >50%, it needs to be tailored to specific environment, w/ specific temps/air circ variables accounted for. can manipulate growth/water uptake from that comfort zone point (~45-65%, again, depending on gardens other inputs).

hope this helps. enjoy your vegetables+garden!

wow good post. How would u tailor a nute formula for a really low rh..? just add less?

And ur saying i can have a higher rh as long as i have good air movement..?

If growth slows in presense of a high rh, then why are we vegging with such high levels?

So ur basically saying that since the plants drink more in a low rh environment, that it increases growth/yield simply cuz the plant is drinking more water than usual..? you said its not taking nutes, mainly water, so how does it benefit from plain h2o..?

Holy Shit Dom Goddess
That was one hell of a post. Great info.
I was gonna chime in myself but........who can top that answer?

thx pirate.

smurfin'herb said:

wow good post. How would u tailor a nute formula for a really low rh..? just add less?

And ur saying i can have a higher rh as long as i have good air movement..?

If growth slows in presense of a high rh, then why are we vegging with such high levels?

So ur basically saying that since the plants drink more in a low rh environment, that it increases growth/yield simply cuz the plant is drinking more water than usual..? you said its not taking nutes, mainly water, so how does it benefit from plain h2o..?

Click to expand...

low rh formula is basically water add-backs, as old lucas formula/threads discussed. was not discussed so much because of rh, but because plants generally utilize more water than nutes during any res/watering/feeding session/change. ppm of nutes will be higher w/ less water in res, if recirculating, but if hand-watering, feed every 2-3 waterings, etc.

plants want 100% humidity in root-zone+100% humidity in atmosphere. they are constantly functioning to balance this ratio out. 100% rh is inapplicable, so gardeners try to get as close to this as possible, w/out incurring disease/rot/pests, etc.

less nute application is best standard formula to begin w/ due to non-linear absorption. different nutes assimilate @ different ph's, etc.

higher rh promotes growth, as plant is not using energy to simply transpire in attempt to balance in/out vpd. as fruiting commences, nutes+light regime+lower rh encourages fruiting. plant has broad comfort range and will adapt to changes within range.

higher rh w/ good air flow is doable. plant will draw up less root-zone water/transpire less. gardeners seek to 'push' the plant during flowering/fruiting by reducing rh for greater nute absoption/water uptake.

yield is inter-related to all garden variables: cultivar, rh(vpd), diff, root-zone temps+mass, light regime(s), etc. hard to standardize yield assesments w/ thousands of cultivars/various garden environmentals.

plants will uptake+transpire more h20(water) during low rh. nute assimilation/absoption is non-linear/non-correlative to water absorption. the plant will not use as much nutes as water during low rh, as plants only have a need for/can only process a limited amount of nutrients. they need water 100% of the time.

pre-mix organic mixes w/ water only throughout the cycle are documented here. possible to use less nutes, w/ more water add-backs, in hydro, w/ success.

if the garden is producing good vegetables, keep the comfort zone you have established for your own particular environment, regardless of what books/threads, etc say. best indicator of good garden is gardener's own pleasure w/ fruits/vegetables.

hope this helps.

great info.
"plants want 100% humidity in root-zone+100% humidity in atmosphere."

I thought that plant stomata completley close at 100% humidity.? maybe im wrong.

plants wants micro-climate in a balanced comfort-zone. they will adapt to different environments, from swamp to desert, etc. plants+humans are constantly adapting to ever-changing environment. humans want salt+sugar, though in excess will cause 'nutrient deficiency'.

water uptake from roots+stomata opening/closing+transpiration are inversely related, but non-linear.
lowest water uptake prior to lights on. stomata closed @ night, partially closed @ mid-day (hottest) so as not to evaporate too much water.

stomata opening/closing is influenced by rh(vpd), temp(diff), light(spectrum(s)), hormones, etc. not isolated to rh influence. lot of science in this. discoveries in plant science/ag eng still continuing. basically, no variable can be read in isolation in garden environ.

most important time to influence/test/experiment on plants w/ different rh/higher night temps/etc. seems to be ~before sunrise/sunset, when the internal clock is most sensitive.

This is great k+. However, I average about 25% in my room because of the air movement and geographic location. I've been told numerous times that RH this low is great, but for some reason I thought that very low RH resulted in reduced yield. Anyone have any knowledge? Just something I've always thought about....

science has been dropped. K+

I've noticed that my plants get more "resin-y" in a drier room as opposed to a humidly run room.

wow- great info. thanks purp

Great info Purp I was wondering about that just today, I think I better go get a humidifier as my RH in veg is at 20%. Defiantly a post worthy of rep.

I am continually impressed with purpledomgoddes' responses. Thanks for taking the time to make such valuable posts.

BowlPacks said:

This is great k+. However, I average about 25% in my room because of the air movement and geographic location. I've been told numerous times that RH this low is great, but for some reason I thought that very low RH resulted in reduced yield. Anyone have any knowledge? Just something I've always thought about....

Click to expand...

w/ low rh(high vpd), just maintain res level/water availability. plants will uptake more water from roots in order to transpire more in attempt to force more moisture (vapor/water/'humidity') into the surrounding atmosphere.
this influences yield due to the increased absorption, and processing of h20+nutes. of course, more water is needed than nutes at this point. be careful during hand-waterings, or res changes to keep ppm's low w/ awareness of plants' usage of h20 during low rh. as for yield increase, faster metabolism generally increases growth/fruiting. rh is one factor, but no factor in isolation will ^yield. light levels+spectrum, temps (day and night), etc.

thx to gardeners for positive responses. enjoy your garden!

purpledomgoddes said:

stomata opening/closing is influenced by rh(vpd), temp(diff), light(spectrum(s)), hormones, etc. not isolated to rh influence. lot of science in this. discoveries in plant science/ag eng still continuing. basically, no variable can be read in isolation in garden environ.

Click to expand...

Effect of night temperature on daytime stomatal conductance in early and late successional plants

A. P. Drew1, 2, 3 and F. A. Bazzaz1, 2

(1) Department of Botany, University of Illinois, 61801 Urbana, IL, USA
(2) Department of Forestry, University of Illinois, 61801 Urbana, IL, USA
(3) Present address: College of Environmental Science and Forestry, State University of New York, 13210 Syracuse, NY
Received: 12 February 1982

Sixteen annuals, biennials, and herbaceous and woody perennials characteristic of early and late successional old field ecosystems and upland and floodplain habitats were analyzed for their response of stomatal conductance to changes in night temperature. Early successional species that germinate in early spring when temperatures are low, but above freezing are insensitive to cool nights, i.e., their conductance in the following days is unaffected by low night temperature. Later spring and summer-emerging species' stomatal conductance is inhibited by low temperatures. Tree species show the same effects and in some an enhancement of stomatal conductance by low night temperatures was observed. However, adaptive differences in response to night temperatures appear related to both phenology of germination and growth and habitat types.

http://www.springerlink.com/content/q775104372hr2476/

Click to expand...

LJB said:

Quote:
Originally Posted by purpledomgoddes
stomata opening/closing is influenced by rh(vpd), temp(diff), light(spectrum(s)), hormones, etc. not isolated to rh influence. lot of science in this. discoveries in plant science/ag eng still continuing. basically, no variable can be read in isolation in garden environ.

Effect of night temperature on daytime stomatal conductance in early and late successional plants

A. P. Drew1, 2, 3 and F. A. Bazzaz1, 2

(1) Department of Botany, University of Illinois, 61801 Urbana, IL, USA
(2) Department of Forestry, University of Illinois, 61801 Urbana, IL, USA
(3) Present address: College of Environmental Science and Forestry, State University of New York, 13210 Syracuse, NY
Received: 12 February 1982

Quote:
Sixteen annuals, biennials, and herbaceous and woody perennials characteristic of early and late successional old field ecosystems and upland and floodplain habitats were analyzed for their response of stomatal conductance to changes in night temperature. Early successional species that germinate in early spring when temperatures are low, but above freezing are insensitive to cool nights, i.e., their conductance in the following days is unaffected by low night temperature. Later spring and summer-emerging species' stomatal conductance is inhibited by low temperatures. Tree species show the same effects and in some an enhancement of stomatal conductance by low night temperatures was observed. However, adaptive differences in response to night temperatures appear related to both phenology of germination and growth and habitat types.

http://www.springerlink.com/content/q775104372hr2476/

Click to expand...

that is why this imaginary garden runs higher temps constantly... @ least 78-88*f.
they love the higher night temps; lets them continue to respirate @ hi rate @ night, as long as keep air circulation on high all (their) night.

purpledomgoddes said:

that is why this imaginary garden runs higher temps constantly... @ least 78-88*f.
they love the higher night temps; lets them continue to respirate @ hi rate @ night, as long as keep air circulation on high all (their) night.

Click to expand...

Do you keep your ventilation running as well? Or just your circulation fans?

This thread just answered sooo many of my questions about enviroment! The calcium bit was very good info for me!
My RH is dead at 20% during winter, now it's around 45% in summer, although if there's no rain it'll go down to around 30%.
Any thoughts on plants smelling more in dry conditions? I dunno why, but my shit stinks like hell in veg as well,
and clones taken to different places do not match the stank in no way.

Excellent info about a lot of different factors. Purpledomgoddes, you impress with what looks like scientifically based information. Where did you learn these facts? Just looking to do some reading of my own, to better understand plant metabolism.

TheMintMan said:

Do you keep your ventilation running as well? Or just your circulation fans?

Click to expand...

sealed room. ac. no intake/outtake. only heat exhaust from condenser side of ac.

air circulation 24/7 in the room is a must. respiration occurs 24/7, even though stomata not open @ night. keep the fans going inside of garden. or if have in/out air, that should be going 24/7. should not turn from 'high' setting to a 'med' or 'low' setting just because it is night for plants.

Sweet Inc. said:

Any thoughts on plants smelling more in dry conditions?

Click to expand...

interesting observation. have not made such a distinction; but have not had idea to look into it. may require more research.

Power13 said:

Excellent info about a lot of different factors. Purpledomgoddes, you impress with what looks like scientifically based information. Where did you learn these facts? Just looking to do some reading of my own, to better understand plant metabolism.

Click to expand...

a lot of studying, application and experimentation.

greenhouse engineering; plant physiology; agricultural engineering; controlled environment agriculture; botany; biology; chemistry; and organic chemistry are a few areas that provide knowledge of these fields. really no quick way to acquire knowledge, but simply going to the local library for an hour or two a week will teach anybody/everybody something new.

college level texts on biology, chemistry and greenhouse engineering/management will go a long way. should be able to find these @ used book stores, or local library, or local college.

a lot of colleges/universities that have cooperative extensions; dept of ag; natl ag library; etc, provide a slew of data too - free to the public.

would begin w/ a college level biology book. really does go into detail about the actual living processes of plants - and all others living orgs that are symbiotic to plant life.

greenhouse engineering/management text should provide lot of technical data; from people that want to be efficient to save coins+produce veggies in a globally competitive market place.

has taken many moons to acquire sense of ease of gardening. have to get your own comfort zone for your own garden.

really have invested years into learning the art. so, there are still trade secrets :smile: that are too hardly earned to just give away. the data is out there though...

enjoy your garden!

*edit*
growing greenhouse seedless cucumbers in soil and in soilless media
[excellent text]