Lighting My Grow

I have a 3ft x 3 ft grow area. How much light do I need? I need to light it at the least cost possible. Thanks in advance for your help.

— Orvin in Utah

Hello Orvin,

The general grow light standard is that you want at least 50 incandescent watts of light per square foot of growing space.

Your 3 x 3 space is 9 ft.², so if you multiply 9×50, you get 450 watts of incandescent equivalent light needed.

You can mix and match the lights you have in order to come up with at least 450 equivalent watts.

Here is a link to how to build your own grow light for less money.


Dr. Grow

Hard Water Problem

I have hard water.  What can I do about it?

— Desert Guy

Hello Desert Guy,

Hard water is definitely a big problem for growers. The very best solution would be to either use distilled water or reverse osmosis water. Because of your hard water, reverse osmosis requires that you also have a water softener, or the R.O. will fail quickly.

If you use distilled or R.O. water, you will need to add back a little calcium and magnesium. You can buy Cal-Mag at any garden nursery, hydroponics store, or grow store. Advanced Nutrients, General Hydroponics, Humboldt Nutrients, Earth Juice Elements, General Organics and Botanicare all make a good Cal-Mag. We use Botanicare. Other companies also make it. Only use 1/8 to 1/4 teaspoon per gallon of water. You may have to do this every time you water.

If you cannot use distilled or R.O. water, at least use something like a Pur water filter or Britta water filter.

We don’t suggest you automatically add the Cal-Mag back in this instance, but keep an eye on your plants for symptoms of calcium and magnesium deficiency. If you see a deficiency, use 1/8 to 1/4 teaspoon of Cal-Mag per gallon of water.

Below are a couple of pictures of what calcium deficiency looks like. Parts of the plant start to die and its growth becomes very stunted.


Dr. Grow


Simple Fertilizer?

I’ve been reading your Growing AutoFlowers book.  You recommend mixing fertilizers from several bottles to get the exact nutrient mix.  Can’t I just use a food for veg and a food for bloom in lieu of buying and mixing from 5 bottles?


Hi David,

You can be flexible about nutrients, but must be careful not to over fertilize.

Yes, in theory you could just buy one bottle of nutrients for veg and one bottle of nutrients for flowering.  Remember, only use 1/4 of the amount recommended on the bottles.

The real trick is knowing when to switch over with your AutoFlower plants.  With regular Indica and Sativa plants, you have to force them into flowering by reducing the amount of light they receive down to 12 hours a day.  When you do this, you are choosing when the plants switch into flowering mode.  Therefore it is easy for you know when to switch fertilizers.

With AutoFlowers however, you never change the amount of light and they start flowering whenever they are ready.  You are not telling them when to flower.  Therefore, you will have to watch your plants closely to see when they are showing signs of beginning to flower.  You can switch over to the flowering nutrients when your classes are starting to show the first signs of flowering.


Dr. Grow

Question About Temperature

Hello guys, how are you? My coltivation grow up, but I would like to know something about the temperature in the groow room, my grow room (wardrobe) actually have 28° degrees C. I didn’t know this before.

I’ve just now see the temperature because I bought the thermometer, Is too much for the plants 28 degrees? and The humidity is 37%. Is good or not? thank you so much, I have the fan but I don’t turn on it every time..just when I stay at home and for 4-5 hours not every day. what can I do? Thank you. Have a nice day!

— Francesco in Italy

Hello Francesco,

Glad to hear that your seeds are growing well.

We think 28 degrees C (82.4 degrees F). is just a little too warm.  It should be closer to 24-26 degrees C (75-78 degrees F) for a good grow.  Maybe you could use a fan more often to cool it down? If you plug the fan into a timer, you could set the timer to go on several times per day.

As long as it does not get hotter than 28 degrees, your plants should do ok.

When the grow area gets too hot, the plants can produce much smaller flowers than normal.

Your humidity at 37% is very good.  It should always be lower than 50% humidity to prevent mold problems.


Dr. Grow

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