There are very few lights out there that we would say will not work as grow lights. Basically, the plants need light and heat, which is what the grow lights are for, so if the light bulb you have can provide heat and light, then it will work as a grow light.
Will it be the best grow light for your indoor hydroponic gardening system? That is another question entirely. We think the best way for you to get the best growth from your plants in your hydroponics indoor gardening system is to invest in a special LED grow light panel that can be programmed with a timer and different light colors in the color spectrum.
However, we also know some tricks for making any light bulb (except black lights- violet lights really won’t do much for any plant…) be a suitable grow light for your needs. So, will a 4200k mh light bulb work as a grow light? Sure- but not on its own. Here are some things you can do to make it happen for you, though.
Set it up in a panel over your gardening area. You want the grow light to be placed above the plants that need the light and heat from the light bulb. If it did not come with a way to angle it in this fashion, then you will want to make some kind of panel or support system to put it over the garden. This can be very complicated and intricate, but it does not have to be. If you do not have the time to make it perfect, then use something simple like cardboard or wire hangers to get the light in the position you need. These supplies may have to be altered and replaced from time to time, but seeing as how these things are easy to come by, that isn’t such a big deal, is it?
Set up a lighting schedule. Plants need light to survive. They need the heat and the light to grow and progress. However, they do not need it all the time. In fact, if they had the light all the time, they would probably dry up ad fizzle out rather fast. That would be sad considering how much work you have already put in to creating and setting up your indoor soil free gardening system. You need to set up a schedule for lighting the plants. Think of how the plants are out doors in their natural setting. They have a schedule. Low to no light at night. Moderate light in the morning. Heavy light in the afternoon. Moderate light in the evening. Little to no light at night again. You will want to mimic a schedule like this. This can be done manually where you come in and turn on and off your light, or it can be put on a timer. Timers are wonderful and make hydroponic gardening a lot more low maintenance, but they can be spendy, so consider whether or not this is an expense you want to invest in. We love them, but if you are not ready to stick with this indoor soil free gardening thing, then that may not be the smartest move for you just yet.
Find a way to dampen the light. 4200 k mh is not the brightest and hottest bulb out there by any means, but as far as grow lights go, it can be overkill if used at full temp all the time. To protect your plants in your hydroponics gardening system from being over exposed to the light from your 4200k mh light bulb, you will want to find a way to dampen and filter this light somehow.
Screens, films, or even simple lamp shades can all do the trick. If there is a way to put it on a clicker to lower the intensity of the light, that would be a more accurate way to go, but all the aforementioned items will get you exactly what you need. To marry this with the advice in item #2, you can have a schedule of dampening the lights at different times in the day. This will allow your plants to have a similar light schedule to what they would have in the wild. There is a reason plants grow well with that waxing and waning light intensity they get from the sun outdoors, so increasing and then decreasing the intensity of light that your plants receive from your grow light bulb can be very beneficial.
Track the progress of your plants. To make sure you are doing the right thing for you plant with the light bulb you use, keep a record of it. Write down where you place the light, what the schedule is, and how you filter that light. Also write down how your plants do. If something isn’t working, then change it up. The best thing to do when starting a system like this is get all your mistakes out of the way at the beginning. Finding out what doesn’t work is a great way to not have to do those things any more. The best way to avoid those mistakes in the future, and remember the things that work well is to write it down. Set aside a journal or notebook or stack of post-its so you can keep track of what your seeds like and what they don’t like.
Remember that no two gardens are the same. You are going to get all kinds of advice from all kinds of people. Some of this advice will be amazing. Some will be annoying. Some will be completely detrimental to you.
Just remember that what works for you may not work for someone else, and what worked for someone else may not work for you. Take all the advice you get with a grain of salt and thank them for their interest in what you are doing.
Ever since I hooked an ignition coil up to a car battery and zapped myself I've been interested in 'how-to' do all sorts of things--I put some of them here.