IPM: Yellowing Hemp Leaves? Lots of Reasons but Always Check pH of Soil and Water

 Yellowing of the lower leaves in hemp plants can be caused by several factors, and it's important to identify the correct one in order to address the issue effectively.

Yellowing Hemp Leaves caused by root zone pH being too high.

We try and provide the optimal growing conditions here at Arendell Hill Nursery in Tallahassee. Yet sometimes, no matter how intentional we are with organic nutrient application, good soil texture, adequate drainage, proper sunlight exposure and continual IPM we still encounter problems with plant growth.

Lately I've been seeing a bit of lower leaf yellowing on some of our container grown hemp plants. I know they have been properly watered and fertilized however the yellow seemed to be persistent.

There are many cultivation variables known to be responsible for yellowing hemp leaves.

Here are some common causes:

1. Nutrient Deficiency: This is often the most common cause. In particular, nitrogen deficiency can lead to yellowing of the lower leaves because the plant moves available nitrogen to support new growth at the top. Other deficiencies that can cause yellowing include potassium, phosphorus, and magnesium.

2. Overwatering or Underwatering: Both of these can stress the plant and cause yellowing. Overwatering can lead to root rot and oxygen deprivation, while underwatering can cause the plant to become dehydrated.

3. pH Imbalance: Hemp prefers slightly acidic to neutral pH levels (around 6-7). If the pH is too high or too low, it can lock out essential nutrients and lead to yellowing.

4. Poor Drainage: If your hemp plant is in a pot or soil with poor drainage, it can lead to waterlogging and oxygen deprivation for the roots, causing leaf yellowing.

5. Pests and Diseases: Certain pests and diseases can also cause leaf yellowing. For instance, fungus gnats can damage the roots and lead to yellowing of the lower leaves.

6. Temperature Stress: If the temperature is too high or too low, it can cause stress to the plant and result in yellowing.

7. Light Burn: If the plant is exposed to too intense light, it can "burn" the leaves and turn them yellow.

In our case the yellowing was a result of our irrigation water's naturally high pH of around 8.0 to 8.5. High pH water can, as mentioned above, prevent the hemp plant roots from taking up nutrients, even if those nutrients are available and exist in plentiful quantities within the hemp plant's root zone.

Irrigation pH balancing involves taking into account acidic rainfall amounts and then pH of natural irrigation water. During droughts, when more higher pH irrigation water from the well is used, more 'pH Down' (a water acidifier we use) must be added to irrigation to keep the root zone pH around 6.0-6.5.

When we have periods of significant rainfall, the hemp plant's root zone is less likely to exhibit a higher pH simply because we are using less well water.

Portable, battery powered pH meters are an important tool for Florida hemp growers (and any gardener for that matter). Here is a link to the pH meter we use.

Remember, identifying the exact cause requires observation and sometimes a process of elimination. Always consider the overall growing conditions including light, temperature, humidity, and the plant's watering schedule. If you're growing indoors, you can more easily control these factors compared to outdoor growing.