Kick-Start Slow Growers

Strategies to help hydroponic plants be more vigorous and productive.


You can’t actually watch your plants grow (at least not without time-lapse photography), but you can tell when they’re not growing. From the beginning, you can observe that they stretch a few inches taller and add new leaves each week. When you don’t see new leaves opening at the tops of your plants for a few days, they likely have stalled. After plants reach their mature height, they switch from growing stems and leaves to flowering. At that point, they are no longer getting taller and bushier. Instead, you should see them forming new flower buds and the existing ones becoming larger and bulkier from one week to the next. If you notice that your plants are stagnating at any stage, what can you do? Check out these factors to get them growing strong again.

Nutrients: Fertilizers for indoor gardens are specifically formulated for each stage of plants’ growth, from seedlings to leafy stalks to flowering. The nutrients in each formula vary based on the plants’ needs during stages of their life cycle. For instance, fertilizers for seedlings or clones are high in nitrogen, which stimulates leaf growth, and potassium, a key nutrient for root development. When plants get ready to flower, they need more phosphorus and less nitrogen. Using fertilizer with the wrong nutrient balance will slow down and may even stop the plants’ growth.

Supplements: Plants produce their own amino acids, the 20 basic building blocks of protein that are involved in many aspects of the growing process. For instance, glycine and glutamic acid help plants maximize the amount of light that leaves can absorb. Others play a key role in stimulating plants’ natural growth hormones. But in times of stress, such as dehydration, the amino acid supply can drop as plants go into survival mode. Giving them an amino acid supplement when their growth has slowed or stopped ensures that all parts of the plant have what’s needed. Phosphorus is the crucial nutrient for flowering. When your plants aren’t producing new buds or adding size and weight to existing ones, you can use bone-meal supplements, organic minerals that are loaded with phosphorus.

PH: The relative acidity or alkalinity of your nutrient solution (fertilizer and water)—its pH—significantly affects your plants’ ability to absorb the vital minerals they need. For example, at a pH below 5.5, nitrogen is “locked out,” meaning that the roots can’t absorb it. At a pH higher than 7.0, plants can’t absorb phosphorus. When plants can’t take up the nutrients they need, their growth stalls. Most hydroponic fertilizers are at their peak of effectiveness when the pH of the nutrient solution is slightly acidic, ideally between 5.5 and 6.5. Be aware that the pH of the water you blend with fertilizers can significantly alter the pH of your nutrient solution. Check each batch you mix up and adjust the pH as needed to be sure it’s in the effective range.

Temperature: Indoor plants grow best when the air and the nutrient solution are between 60 degrees Fahrenheit and 75 degrees Fahrenheit. Cooler temperatures slow the growth of seedlings as they try to simply survive the less than ideal conditions. Hot conditions during the flowering stage can thwart the formation of buds and can also cause rapid evaporation, leaving plants thirsty and impeding their growth. For steady, sustainable growth, use fans and ventilation to keep the room and the nutrient solution no warmer than 75 degrees Fahrenheit when the lights are on. And make sure that the temperatures don’t drop below 60 degrees Fahrenheit when they’re off.

Humidity: While your plants are adding lots of stems and leaves—the vegetative stage—they grow most vigorously when the humidity level is above 40 percent. When it drops to 25 percent or lower, your plants may look like they’re suffering from nutrient deficiencies and they may barely grow. A humidifier helps maintain the air’s moisture content, ideally at 60 percent, even in winter, when the air inside homes tends to be dry. Just remember to dial back the humidity level to around 40 percent to 50 percent during flowering to prevent mold from spoiling the buds.

Oxygen: Microbes help break down organic fertilizers into nutrients that plants’ roots can absorb, and those microbes need oxygen to fuel their feeding and reproduction. Without oxygen, your plants don’t get the nutrients they need to grow and can drift into a holding pattern. If you don’t have an air stone or other pump in your nutrient solution reservoir, you can add oxygen simply by stirring the fluid well once or twice daily.

Carbon dioxide: Plants require CO2 to sustain steady growth. Outdoors, the air has plenty of carbon dioxide for healthy plant growth, but the levels in indoor grow rooms can drop below the 300 to 500 parts per million found in the atmosphere. Ventilating and circulating the air indoors is the simplest way to provide enough CO2. With a carbon dioxide generator, you can give slow-growing plants even more, which will increase photosynthesis and stimulate new growth.

Lighting: Compact fluorescent lights appeal to indoor growers because they are inexpensive to buy and use. They generate sufficient light to grow leafy crops, but they need to be as close as 3 inches above the plants. Light-emitting diode lights can bring flowering plants to maturity, and they are effective when they are about 24 to 30 inches from the top of the plants. When the lights are not strong enough for your crop or are too far away, your plants’ growth can slow or they may grow taller but more spindly. If you find that your plants consistently stall before they reach maturity, you might consider investing in high-intensity lights, such as metal halide or high-pressure sodium, the most effective types for indoor growing. No matter what lighting you use, change the bulbs regularly—the light weakens before your eyes notice that it has dimmed.