Marijuana Grower's Handbook

by Ed Rosenthal

  Symptoms

  The first signs of S deficiency are yellowing, young leaves. Leaf growth is slow; leaves become brittle and narrower than usual, and are small and mutated. Buds die off at the tops of flowering plants. Overall growth is stunted. Some S deficiencies may show orange and red tints rather than yellowing. In severe cases the veins of the growing shoots turn yellow with dead areas at the base of the leaf where the blades join. The stems become hard, thin and may be woody. They increase in length but not in diameter.

  Too much S stunts the plant and leaf size, and the leaves look brown and dead at the tips. An excess of S looks like salt damage: restricted growth and dark color damage. This is also rare.

  Like iron (Fe), S moves slowly in the plant. Warmer temperatures make S harder for the plant to absorb. But unlike Fe, S is distributed evenly throughout the plant, mainly in the big fan leaves. S deficiency starts at the back of the leaves and creeps towards the middle.

  Role in plant nutrition

  Sulpher is essential during vegetative growth and plays an important role in root growth, chlorophyll supply, and plant proteins.

  Problem Solving

  Both organic soils and inorganic fertilizers contain high levels of available S so plants are not likely to suffer from a lack of the element. However, a deficiency is easily solved using Epsom salts (MgSO4). Water the plant with Epsom salts until the condition improves. Mix one to two teaspoons of the salt per gallon (1.3-2.6cc per liter) and apply both foliarly and to the irrigation water. Adding nutrients containing S fixes the deficiency. Mix at recommended strength to avoid nutrient burn. Any water-soluble fertilizer that uses S in the trace minerals also works. Other sources are elemental garden S, potassium sulfate (K2SO4), and gypsum. Do not use gypsum on acidic soil (pH less than 5.5); it affects the absorption of soil aluminum, which is poisonous to plant roots.

  Sulfur deficiency. Photo: Anonymous

  ZINC (ZN)

  Zinc (Zn) deficiency occurs occasionally.

  Symptoms

  New growth has radically twisted leaf blades. Zn deficiencies are identifiable by spotting, chlorosis, and yellowing between the veins of older leaves. Inter-veinal yellowing is often accompanied by overall paleness. During the flowering stage, buds may contort, twist and turn hard. When the deficiency first appears, the spotting can resemble that of an Fe or Mn deficiency but it affects the new growth. Zn excess is very rare, but produces wilting and even death in extreme cases.

  Zn is not mobile in plants, so symptoms occur mainly in the newer growth.

  Role it plays in plant nutrition: Zn aids in plant size and maturity, as well as in the production of leaves, stalks, stems, and branches. Zn is an essential component in many enzymes and in the growth hormone, auxin. Low auxin levels cause stunted leaves and shoots. Zn is also important in the formation and activity of chlorophyll. Plants with high levels of Zn can tolerate longer droughts.

  Problem Solving

  Use an iron-zinc-manganese (Fe-Zn-Mn) micro mix to solve the deficiency. Zn sulfate (ZnSO4), chelated Zn or zinc oxide (ZnO) also adjust the deficiency.

  General Discussion

  With low levels of Zn in the plants, the yields are dramatically reduced.

  TEMPERATURE, HUMIDITY, AND AIR QUALITY

  Marijuana grows well in moderate temperatures – between 70° and 85° F (21°-29° C). Both high and low temperatures slow marijuana’s rate of metabolism and growth. Plants grow fastest when the temperature during the lighted period is kept between 72° and 77° F (22°-26° C). When CO2 is being used, the plant prefers to be a few degrees warmer, between 79° and 85° F (26°-29° C). Individual marijuana varieties differ in their temperature preferences by a few degrees, so some experimentation is required to find the ideal temperatures for the strain you are growing.

  Ideal temperature is tied to light conditions. As more light is available, the ideal temperature for normal plant growth increases. Strong light and low temperatures slow growth and decrease stem elongation. Conversely, when plants are given high temperatures and only moderate light, the stems elongate.

  Plants growing under moderate intensity lamps (40-50 watts per sq ft, 430-540 watt per sq m) should be kept on the low side of the recommended temperature range. Plants growing under higher intensity lamps (60 watts per sq ft, 650 watts per sq m and higher) should be kept on the warmer end of the scale.

  During dark periods, the temperature can be kept as much as 10˚ F (5˚ C) cooler than the lit period without any negative effects. Wider temperature differences cause slower growth, stem elongation and delayed flower ripening.

  Plants that are kept at a constant temperature are likely to grow stouter, sturdier stems and have denser bud growth. Plants that experience a large differential between day and night temperatures suffer from stretching and slowed growth rates.

  At temperatures below 60° F (15° C), photosynthesis and plant metabolism slow, stopping growth as it waits for better conditions. As soon as the temperature rises, the plant resumes full functioning. When the temperature falls below 40° F (4° C), marijuana plants experience tissue damage and require about 24 hours of warmer conditions to resume growth. Young marijuana plants are somewhat tolerant of low temperatures; when outdoors, seedlings have been known to pierce snow cover without ill effect. But low temperatures during ripening, even just overnight, delay or prevent bud maturation. Some equatorial varieties stop growth after a few nights with temperatures below 40° F (4° C).

  TEMPERATURES PLANTS LIKE

  Germination: 70-78° F (21-25° C)

  Vegetative: 68-82° F (20-27° C)

  Flowering: 68-80° F (20-26° C)

  Cloning: 75-85° F (24-20° C)

  EXCESSIVE HEAT

  Marijuana plants are very hardy and survive outdoors over a wide range of temperatures. They can withstand extremely hot weather, up to 120˚ (49˚ C) for short periods, as long as they have adequate supplies of water and a large root system.

  As the temperature rises from the high 70’s into the 80’s F (20-25˚ C), plants spend more energy staying cool and maintaining faster cell metabolism. However, under high light conditions, photosynthesis increases as the temperature rises, resulting in a net gain in plant growth. Photosynthesis reaches it apex at about 85° F (30° C) and between 6000- 7500 fc (81,000 lux) of light. However, as heat rises further to 90° F (32° C), photosynthesis slows until it stops at about 95° F (35° C). At this point, plants go into preservation mode; photosynthesis stops as the plants spend energy acquiring water and transpiring it through the stomata in order to keep cool.

  Outdoor plants facing long, hot spells should be well watered so the roots can draw upon adequate supplies with little effort. As the soil dries out, water tension increases, and the soil holds on tighter to the remaining water. Keeping the soil moist makes it easier for the plant to draw water from the medium. Plants can also be protected from the heat using a spray-on anti-transpirant, which keeps the plant from shedding water. Make sure to use a non-toxic spray that is listed as safe for use on edibles. Still, it is questionable whether sprayed leaves should be used.

  Plants photosynthesize more rapidly in high light-high CO2 conditions when the temperature is around 85° F (30° C)

  Excessive heat can be a problem indoors too, though it is easier to control. Gardens using high wattage lamps generate a lot of heat; an unprotected 1000w metal halide and ballast emits about 3500 BTUs, while air-cooled lights generate about 500-1500 BTUs. During the winter, the heat produced may keep the garden space comfortable, but during the summer, the space may get too hot, particularly if there are several lights heating up a room.

  The temperature of the uppermost foliage is the specific area of interest. The space in the aisles or the floor may be cool, but that doesn’t matter to the plants. What is important is the temperature of the canopy under the lights where the plants are producing new growth. Even when the overall temperature of the room is in the optimal range, the areas directly under the bulbs can be very
hot.

  EXCEL AIR SYSTEMS DIY AIR CONDITIONING UNIT—Photosynthesis quickly slows to a stop as the temperature climbs past 90° F (32° C). The Excel Air AC Unit allows you to preset and regulate the temperature and also has the ability to incorporate other essential grow room elements such as CO2, odor eliminators and air purifiers.

  A surface thermometer measures the temperature on the leaf surface. It is the best indicator of the temperature that the plant is experiencing. To use point the infrared thermometer at the leaf or other surface measure, and it will give a digital readout. When using conventional ther-mometers, make sure to place them at the top of the plant canopy; this provides an accurate reading of conditions in the growing area.

  All ventilation systems should have an intake filter to prevent insects and mold spores from entering the space. This Dust Shroom filter cleans incoming air. Notice the pests and dust on the filter surface.

  COOLING INDOORS

  There are several ways to manage heat in an indoor garden space:

  •Don’t create a hot environment. Use air or water-cooled lights to prevent heat from being trapped in the garden space. Keep heat-producing ballasts outside the growing area.

  •Run lights at night. If the room is lit exclusively by lamps, the day/night cycle can be reversed so heat is generated at night, when it is cooler outside.

  •Vent the garden area with filtered air. During the winter and in the evening, outdoor air may be cool enough to lower the temperature of the garden.

  •Install an air conditioner. Air conditioners can be set up to exchange the heat in the room without venting telltale odors. Portable air conditioners work well, and window models can be installed in a window or an internal wall to vent the heat into a central area.

  Properly positioned fans in the grow room cool the air heated by the lights, strengthen the stems, and help prevent pests from landing and clinging to your plants. The Max-Fan from Can Filters is small, energy efficient, and quiet.

  •Use a cooler. Warm dry spaces can be cooled using portable air coolers. These appliances cool the air using evaporation. Air is drawn through a wet filter; as the water evaporates, it cools the air.

  Even without lights, an enclosed space can get hot rapidly when outdoor temperatures rise. Greenhouses get very hot during the summer as bright sunlight heats up the interior and the roof traps the heat, so the temperature increases throughout the day. Some greenhouses have roofs that open to let hot air escape and draw cooler air in.

  This portable evaporative cooler contains its own reservoir. It’s easy to move to spaces where it is needed.

  For closed greenhouses, swamp coolers are very effective. Water runs through fibrous plastic mats as fans blow air, lowering the greenhouse temperature. Because swamp coolers work by evaporation, they are most effective in hot, dry areas.

  Another evaporation technique uses five-micron spray nozzles, or cooling fans, to pulverize water into small particles. The water pieces are so small that they immediately evaporate as they are sprayed into the hot air, lowering the ambient temperature. Spray coolers are available through nursery supply houses.

  Plants stop photosynthesizing and buds grow lanky when the temperature gets much above 80-85˚ F (27-30˚ C). A misting fan (circled) blows 5-micron droplets of water into the air that quickly evaporate, bringing the temperature down by 20˚ F (11˚ C) and getting the plants back to work. Strings of 5-micron misters (not pictured) work the same way.

  COOLING OUTDOORS

  Photosynthesis crawls to a halt when the temperature gets close to 90° F (32° C). In hot weather outdoor plants may be photosynthesizing efficiently during only part of the day. For instance, if the temperature climbs to 90° F (32° C) at 11 a.m. and does not drop below that level until 4 p.m., the plant will utilize only the morning and late afternoon light. High temperatures outdoors during flowering interferes with bud development, making maturing buds airy and lanky. Temperature is a factor when plants are forced to flower in hot areas during the summer.

  Fortunately, there are several ways to keep plants cool outdoors:

  •A fine spray of water on the plants keeps them cooler during the hot period. The evaporating water absorbs heat from both the leaf and the air in the micro-space at the leaf surface. Use a water spray only during vegetative growth and the earliest stages of flowering. Spray early enough in the day so that all the water has evaporated before dusk.

  •Misters that use 5-micron sprayers evaporate water in the air, cooling the space without getting the plants wet. Some systems use rows of emitters, high-speed fans, or a combination of both to create a super-fine mist or fog that evaporates instantly, lowering the temperature as much as 30˚ F (16˚ C). They work best in dry areas.

  •Outdoor fans and misters.

  ROOT TEMPERATURE

  The temperature of the plant canopy is critical, but it is not the only area to consider. Root temperature is also important. Cold floors lower the temperature of containers and the planting medium, slowing germination and growth. Cold temperatures also encourage more of the plants to develop as males when you are growing from seed. When a plant’s roots are kept warm, the rest of the plant can be kept cooler with no damage. Ideally, the medium temperature should be 70° F (20° C). There are several ways to warm the medium, or protect it from cold surroundings:

  •The best way to insulate a container from a cold floor is to raise it so there is air space between them. A thin sheet of Styrofoam, foam rubber, wood, or even newspaper between the container and the floor serves to insulate.

  •The medium can be warmed using overhead fans to push the warm air down from the top of the room, warming containers that are placed on pallets.

  •Heat cables, or heat mats, apply heat directly to the root area.

  •Heat the water in re-circulating systems with an aquarium heater controlled by a thermostat. If the air is cool, from 45°-60° F (27°-15° C), the water can be heated to 80° F (27° C). At these high temperatures, hydroponic system water should be supplemented with oxygen using hydrogen peroxide. Water holds little oxygen above 80° F (27° C).

  Humidifiers keep the humidity of clone rooms at the desired 75-80%.

  In general, water temperature should be adjusted to balance out the air temperature. If the air is warm, over 75° F (22° C), the water should be no more than 70° F (20° C). Should air temperature rise above 90° F (30° C), lower the water temperature from 70°-65° F (20°-18° C) to help decrease canopy stress.

  HUMIDITY

  Cannabis grows best in an environment that is mildly humid: 40-55 % relative humidity. Plants growing in drier areas may experience a slight chronic wilt and necrosis of the leaf tips. Plants growing in a more humid environment usually experience fewer problems, but the buds become susceptible to molds that can attack a garden overnight and ruin a crop.

  Indoor gardeners are rarely faced with conditions that are too dry. In enclosed spaces, water which is evaporated or transpired by the plants contributes to the humidity. With no ventilation or other humidity control, the space, whether large or small, gets excessively humid within hours.

  The solution may be as easy as opening a window or adding a small ventilation fan. A dehumidifier can also resolve the problem in closed environments.

  If the air in the room becomes humid, marijuana crops run the risk of becoming moldy. Products that maintain pre-set climate levels are helpful in areas with extreme temperatures and high or low humidity. The Novabiomatique Plug ‘n’ Grow controls the temperature, humidity, and CO2 concentration of the room as well as allowing the gardener to prioritize the actions in a logical sequence, and efficiently adapt to local climate conditions.

  Dehumidifiers work the same way as a refrigerator but instead of cooling a space, moisture in the air condenses and collects on the cold tubes. A dehumidifier may be needed only a few hours a day. If the plant regimen includes a dark cycle, then the dehumidifier can be run when the lights are off, easing the electrical load. The smalle
st dehumidifiers (which can dry out a large space) use about 15 amps.

  AIR CIRCULATION

  Good air flow within the growing space is important to the plants’ health. Plants depend on the air movement to grow strong and vigorously.

  Without air currents, the leaves’ rough surface and tiny hairs trap the air in a micro-environment that differs significantly from the surrounding atmosphere. The plant uses CO2, and produces oxygen and water which depletes CO2 in the air surrounding the leaf, but contains more humidity and oxygen than the rest of the space. Until this air is replaced with new CO2 laden air, photosynthesis slows.

  Marijuana depends on air currents to move air and renew the micro-environment. When it is not moved vigorously, the growth rate slows and the micro-environment remains CO2 depleted.

  Vigorous air movement mimics the natural environmental stresses that encourage plants to develop firm, sturdy stems. As plants sway with the wind in an outdoor environment, they develop tiny breaks in the stem. The plants repair the damage quickly by reinforcing the breaks and tears, leaving them stronger and stouter than they were originally.

  Indoors, plant stems grow weak unless the plants have air flow to resist, or are shaken by the stems daily.

  ASK ED: Marijuana Questions

  ICE WATER FOR PLANTS

  The garden’s water source is drying up and won’t last through July. The plants are in a field in the sun and it gets really hot here, over 100 ° F (38˚ C) every day during July and August. Can I place ice at the base of my plants or will the cold water have some effect on the roots or growth of the plant? It’s easier to carry bags of ice then buckets of water.