Basil Leaves Turning Brown? Here’s Why

Basil is commonly referred to as the king of the herbs, as it is so widely used for culinary purposes. We have grown a wide variety of basils, from classic Italian Genovese, to sweet Thai (a personal favorite), to lemon and cinnamon basils. Needless to say, it’s a popular herb for home gardeners.

While it is easy to bring home a healthy-looking basil plant from the nursery, it is just as easy for the plant to decline. One of the most common issues is basil leaves turning brown.

So, in this article, I’ll cover 7 of the most common reasons your basil leaves are turning brown, and what you can do about it. Most of these problems can be remedied with small adjustments to your plant care routine. Let’s get into it!

Brown spots on basil leaves

1. Low Temperatures

The most likely cause for your basil leaves turning brown is cold temperatures. Basil can be very sensitive to temperature fluctuations, especially those below 50°F or so.

Basil is native to sub-tropical Asia (Thailand, Pakistan, India, etc.), and is not adapted to colder climates. So, if your basil plant experiences a cold overnight, it may become very stressed.

It is fairly common in temperate climates (such as ours in New England) for overnight temperatures to start dipping in late summer. So, to avoid brown basil leaves, check your weather forecast for overnight lows below 50°F or so.

If you are expecting a cold spell, bring your potted plant indoors overnight. You can also cover any plants in the ground with a row cover or even a 5 gallon bucket if the plants are small enough. This will insulate the leaves and prevent stress and discoloration.

2. Lack of Pruning

One of the most important routines when growing basil at home is pruning. Pruning basil can lead to much bigger overall yields, and can keep your plants looking bushy and healthy.

Basil flowers and dying leaves
When basil begins flowering, it should be pruned to encourage more leafy growth.

In essence, the goal of pruning is to remove any flower shoots from the plant. This redirects energy towards more leafy production instead of flowers and seeds.

Not only does this produce a larger yield, but it can prevent the existing leaves from turning brown and pale. Once a basil plant enters its flowering period, the foliage becomes less important. Instead, the plant will focus energy on seed production.

So, harvest your basil regularly, cutting above a node on each stem. This, coupled with plenty of nitrogen in the soil, re-invigorates the plant to grow more healthy, green foliage.

3. Disease

Like any plant, basil can be susceptible to disease. Fungal pathogens are everywhere in environment, both soil and airborne. Downy mildew, fusarium and other pathogens can infect basil plants, causing brown or black leaves.

Basil in window
Basil with fungal infection on lower leaves.

The chances of getting disease is made worse in unfavorable conditions. For example, over-crowded plants with poor airflow will be more likely to become diseased. Also, pest-infested basil plants will be an easier target for disease to take hold.

The best way to avoid disease on your basil plants is to plant disease resistant varieties. Since basil is so popular, there are several hybrid varieties with disease-resistant properties. For example, Prospera F1 basils all carry fusarium and downy mildew resistance. Look for the acronym “DMR” when buying basil seed, which indicates downy mildew resistance.

Tips to avoid disease:

  • Plant disease-resistant basil
  • Don’t water over the tops of leaves
  • Mulch the base of your plants
  • Space plants properly for good airflow
  • Control pests
  • Remove dead or diseased foliage

Common basil diseases:

Fusarium wilt is a particularly common and devastating disease of basil. The symptoms may not appear until the plants are 8-12 inches tall. Once fusarium sets in, the lower leaves will begin to brown, and the plant will wilt and eventually die.

Downy mildew is the other most common disease of basil. This infection causes yellowing on the tops of leaves, and a brownish-black fuzzy coating underneath. Downy mildew starts on lower leaves and spreads upwards over time. Avoid watering over the tops of leaves, and bring plants indoors if possible during humid or rainy weather. If you have an infected plant, remove it from the garden to prevent spread to other basil plants.

4. Over-Watering

Another common mistake is over-watering basil. Your basil plants do like an evenly-moist soil, but too much water can become a major problem. This is especially common if your soil has poor drainage.

Before you water, make a point to check below the surface of the soil, at least 2-3 inches down. If you feel or see moisture in that soil, hold off on watering. Also, make sure any potted plants have at least 1 drainage hole in the bottom to allow water to flow out.

The reason your basil leaves might turn brown from over-watering is a lack of oxygen in the soil. Roots require oxygen to “breathe,” and without it, they can rot. Root rot is common in wet climates, or in places that have heavy clay soil.

The best way to avoid overwatering is to water only as needed, plant in a well-drained soil, and amend clay soils with compost to improve drainage.

5. Leaf Scorch (Sun Scald)

Early in the season, your young basil seedlings are tender. This is especially true if you’ve grown them indoors from seed. If you move them out into the sun suddenly, they may get a sunburn.

In the gardening world, this burning is called leaf scorch, and is caused by direct sun exposure to tender foliage. The leaves usually turn white at first, eventually going brown and becoming crispy and dry to the touch.

To avoid leaf scorch on your basil, make sure to adjust your seedlings to sunlight gradually. When transplanting basil outdoors, give the plants progressively more time in the sun until they can handle it all day long.

6. Pests

In general, basil is fairly pest resistant. In fact, is makes a great companion plant for other garden plants. However, that does not make basil immune to all pests.

If pest populations get large enough, you may notice aphids, thrips or whiteflies feeding on your basil plants. This can cause damage to the thin basil foliage, turning them a brown color. These pests typically cause damage near the tops of the plants, where the newer foliage is just forming.

To deal with pests:

  • Plant flowers in your garden. This is the best way to keep pest populations at bay. Flowers attract a variety of insects, including beneficial ones. These include lacewings, hover flies, and ladybugs. Some of the best companion flowers include alyssum, cosmos, asters, and marigolds.
  • Spray off with the hose. A strong jet of water will knock off a majority of aphids on your plants. Be thorough, and ideally do this away from your garden (if your plants are in pots, for example). This won’t completely get rid of the pests, but it can set them back considerably and avoid further damage.
  • Spray with insecticidal soap. There are many organic pesticide options on the market that are safe to use on food crops. We like this brand from Amazon, just make sure to use it only as a last resort when pests are out of control. These products can kill beneficial insects, too.
Closeup of hover fly on alyssum
Hover fly on sweet alyssum plant.

7. Lack of Nutrients

The last possible cause of basil leaves turning brown is a lack on nutrients. Certain nutrient deficiencies can lead to a dark discoloration of foliage.

Basil leaf with brown spots
Nutrient deficiency on hydroponic basil leaf.

A simple research study was conducted to show common nutrient deficiencies in hydroponic basil. Nitrogen, sulfur, and iron deficiencies lead to pale, yellowing basil leaves, while the following nutrient deficiencies showed browning.

Nutrient deficiencies that can cause brown leaves on basil:

  • Phosphorus
  • Potassium
  • Calcium
  • Magnesium

If you are growing in soil, it is less likely that you are dealing with nutrient deficiency. However, if your plants have not had any supplemental fertilizer, it can help to feed your basil.

We use an all-purpose fertilizer to keep our herbs growing lots of healthy green leaves. Since the leaf is what we use from basil, it is best to use a fertilizer with a higher nitrogen content. Avoid fertilizers labeled for “flowering” or “blooming” stages of growth.

Hopefully this post helped you to solve the mystery as to why your basil leaves are turning brown. As you can see, there are many reasons this may happen. Basil continues to be one of our favorite herbs to grow in the garden and good with in the kitchen.

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