What are common mistakes made by first time growers?

First time growers frequently:

Overwater their medium based plants. When you pot your plant, judge its weight dry by hefting. Then, water the plant thoroughly, until water runs through the drain holes. Heft it again. When your pot feels as nearly as light as it did dry, its time to water again.
Overfeed their plants. As MrSoul says, beginners rarely under do anything. When all else fails, follow the directions on the fertilizer bottle.
Overanalyze their grow. A first grow is like a first born child: you pay attention to every little thing that happens. Further, you worry at the first sign something may be wrong. Pay attention to what happens in your grow, but do not try and find a remedy for every yellow leaf. Frequently, the remedy-like flushing your medium-causes more problems than it solves. Be responsive when things go wrong, but be conservative in your remedies.
Overspend on grow supplies. New growers frequently commit themselves to unrealistic and expensive first grows. It is much more efficient to learn to grow and then invest in high end equipment rather than the other way around. Most experienced growers don't have room for all of the grow paraphernalia they aren't using. As you will see throughout this FAQ: K-I-S-S.
Overpost. Try the search engine first. Chances are good that any question you can think of has been asked and answered before.
Talk about their grow. Don't tell anybody that does not have to know. How can you expect anybody else to keep a secret that you cannot.

Part 2: Contributed by: BushGrower

1. Don't Overwater Overwatering kills marijuana plants. Water once the top few inches of the soil dries out. Hydroponics is harder to over water than soil, due to the abundance of water roots.
2. Don't Tell People Why? They will only be jealous. People love to feel important and that is why they will tell other people; because others will listen to them. Keep it to yourself.
3. Touch/kill Germinating Seeds It can take up to 10 days for a seed to sprout. The paper towel method is not recommended because you must handle the seeds when transferring them from the paper towel to your growing medium.
4. Grow seeds from seeded marijuana (hermaphrodite seeds) Unless you are prepared for possible disappointments don’t use "unknown" seeds. This is why people buy seeds from seedbanks. Self-seeding MJ is produced from hermaphrodite plants or a very stunted and late flowering male the grower did not notice. Flowered hermi seeds will produce tall late flowering females coupled with early flowering males.
5. Don't Over-fertilize Fertilize after first 2 spiked leaves appear. Start with 25% of recommended label strengths and work your way up. If the leaves suddenly twist or fold under, Leach and Spray with pure water for several days! Don’t fertilize your plants every time you water! A common watering schedule is to fertilize at full strength, then water at half or quarter strength. This prevents excess salt buildup, leaf and root burn. In addition, don’t water at full strength if the medium is too dry – root burn can occur. As a precaution, leach the plants with lots of pure water every 2-4 weeks.
6. Don't Under-fertilize Under-fertilizing is less common. If you prefer to give the plant ‘just enough nutrients’, use a organic soil mixture with blood meal and bone meal or some slow release fertilizer with micro nutrients.
7. Don't Start with Clones Start with seeds. Bugs are a pain, as are plant diseases. Many growers are able to grow indoors without pest problems for years. Another grower’s cuttings are almost guaranteed to have diseases &/or pests.
8. Don't Provide A Bad Environment Always provide air circulation and fresh air even during the night cycle. All the air indoors should be replaced every 5-10 minutes. Humidity between 30-70% temp aim for around 75-85' Even seedlings need a gentle fan to strengthen the stems.
9. Don't Harvest Too Early 25% of the weight will form in the last 2 weeks. Begin flushing with 100% pH’d water when the pistil are 25% brown. Harvest when the plants have totally stopped growing and the white pistils are at least 50-75% brown.

* NOTE: Outdoors if security is a factor make your own call on when to sacrifice the fields. Also take buds continuously in case of thieves.

Common questions:

Q. Can marijuana grow in a northern climate? Marijuana plants can grow anywhere corn can grow. All it needs is three growing months - seed to harvest. 2 if started indoors!
Q. Why do I have to buy seeds? Why can't I use my own that I picked from my own stash? Most people desire, and want to be guaranteed, certain characteristics in their mature female plants. The seeds from any weed will all grow into something different. This is unprofitable and inefficient. As opposed to knowing the single set of requirements for your entire crop, you must provide a different set of requirements for each of your plants.
Q. What is better for a new grower - hydroponics or soil? I believe the all around "better", more convenient setup is soil. Hydroponic grows faster, yet does not produce extra potent buds. Hydro should be attempted after you have a few successful soil crops under your belt. If you are starting from seed and growing for personal, soil is the practical growing medium. If the crop is started with clones and is commercial, a hydroponics setup is more practical.
Q. Why are my seedlings stretching? Low light conditions. They also need a gentle wind. Plants will also stretch when subjected to conditions of high humidity.
Q. What kind of lights should I use? 
Cheap 4 ft. cool white fluorescent tubes : for germination/seedlings 
400 watt Metal halide/HPS : for personal home growers 
1000 watt Metal halide/HPS : for some personal growers and commercial growers.
* Use at least 40 watts per sq. foot of grow space.
Q. How far should the lights be from the plants? Fluorescent: tips of leaves almost touching bulbs 400 watt halide : two feet away from seedlings and one foot away from grown plants 1000 watt halide: four feet away from seedlings and two feet away from grown plants
Q. How often should you water? 
Once a week or once every two week for soil and twice a day with a hydroponic flood and drain system.
* When top 2 inches of the soil dry out.* Occasionally provide periods of extra dry and wet soil.
* Allow 10% extra water to drain out of the bottom of the tray.* This will prevent toxic fertilizer build up.
Q. How long do your seeds last? What's the best way to keep them? Seeds can last over 5 years if kept cool and dry. They may last up to 10 years if sealed and frozen, but fewer will germinate. Last modified: 07:20 - Feb 25, 2001Login to Rate this FAQ