How Much Light Do You Need?

Grow lighting is something that many new indoor gardeners tend to cut corners on. Yes, it’s important to conserve on energy use, but inadequate light actually makes your food production more expensive. You have to have enough light power so your plants have the energy they need to produce a great harvest. If you don’t, your garden will produce poor to mediocre results. No doubt you’re looking for a bumper harvest, and not slim pickings.

So, how do you know when you have the right amount of light?

What you want to grow will create some guidelines. If you just want to grow some lettuce or greens, and a few leafy herbs, then you will be fine using LED lights. For peppers, tomatoes, eggplant and other fruiting crops – you have to have more light power than that.

The best harvests will always come from growing under HID lights. You will have some success with the lower power consuming high output T5 fluorescent lights if you have enough wattage running, but nothing like the more powerful grow lights will give you. When growing fruiting crops under T5 lights, you absolutely must have highly reflective surfaces above, under and surrounding your plants. And if you’re growing tall plants like tomatoes, plan on adding side lighting when your plants get tall. If you don’t the top foliage will block too much light from the lower parts of the plants.

What Is the Best Grow Light Power?Even when you’re using HID grow lights that surrounding surface reflection is important. Unlike the sun, your grow lights aren’t moving from low horizon to low horizon across the top of the grow room every day. It’s stationary placement means that big portions of your plants’ leaves aren’t able to collect the light energy they would outdoors. Consider how much activity goes on beneath the tops of the trees in a forest. You find the same situation taking root in your grow room when the lower parts of your plants get inadequate light. Reduced flower to fruit action below the canopy will always precede thinning foliage, which takes time, and your grow room crop won’t be standing for a few decades to hundreds of years. Still the effect is very similar.

Fruit and veggie plants are all sun lovers, so you have to do everything in your power to give them light energy where they need it. The entire outside a plant has light receptor leaves ready to go to work for you, but they need good light. With less powerful T5 lights and a tall crop adding side lighting with CFLs or vertically hung T5 fixtures to the walls will increase production very nicely.

Next the dimensions of your grow space set a guideline for how much wattage you need. Don’t go overboard. That is definitely wasting energy, as well as creating stressful conditions for your plants. The following gives you the total wattage you need in HID grow light power for different growing space dimensions. Configure your lights to fit your space appropriately.

250W = 2 – 1.5 ft² (0.6-0.45 m²)

400W = 2.5-3.3 ft² (0.75-1 m²)

600W = 3.3-4 ft² (1-1.2 m²)

1000W = 4-5 ft² (1.2-1.55 m²)

1500W = 5-6 ft² (1.5-1.8 m²)

Finally, the hotter the light, the more distance between lamp and plants’ is needed. Too close, and you’ll have problems with burning on both leaves and fruit. Depending on the crops you choose to grow, you might need to use a less powerful light to keep good distance between canopy top and the light. This will be especially true if your grow room has a low ceiling and you are trying to grow tall plants. The list below gives you best results guidelines in inches and centimeters.

250W = 12-8 in (30-20 cm)

400W = 24-16 in (60-40 cm)

600W = 31-24 in (80-60 cm)

1000W = 39-31 in (100-80 cm)

1500W = 35-45 in (89-114 cm)

Exceptions to these light-to-plant distance rules would be if your vertically mounted side lights have parabolic reflectors, or your overhead HIDs have air cooled reflectors. In this case, your lights are producing less radiant heat allowing for positioning them closer to your plants without burning them.

If you’re using T5 fluorescents you will want to keep the lights at 6-12 inches above the plants. Too much distance with these and the light energy is reduced for your crop.

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Tammy Clayton

Contributing Writer at Garden Culture Magazine

Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home.