Reviving Your Plants: Troubleshooting and Fertilizing Tips

Signs of Plant Stress: Wilted Leaves, Dry Tips, and Pale Appearances

This summer I started a long-distance move and after an extended period away my greenhouse plants are crying - yes crying - for help. Wilted plants, dry leaf tips, pale leaves and poor appearance. The trouble, it turns out, is that my house sitters were overly cautious about watering and using the fertilizer I left out for them. Now that I am home, I am growing my plants back to perfection.

Maybe your greenhouse plants also have small, pale yellow or dull and lifeless leaves? These signs might mean your plants need more fertilizer and nothing is gained by holding back and depriving them. 

When I returned from my trip, the whole plant was pale. After fertilizing the new leaves on the right are green and shiny. This indicates they are perfectly healthy now.

The Importance of Fertilizing Greenhouse Plants for Optimal Growth

Plants grown in greenhouse gardening “soil mix” instead of rich natural soil, need to be fertilized more. No fooling around. Yes, plants make their own carbohydrates from the energy in the sun, but, like a teenager on a growth spurt, plants need protein, or at least the components of protein from fertilizer - especially when they are growing so fast in a greenhouse.

I am reviving my plants after their low-fertility house-sitting experience and, thankfully, the plants in my greenhouse are responding quickly and things are turning around. Your teenager can’t grow a new arm when you offer them a second steak, but your plants can bush out, bloom more and look healthier once you start to fertilize them into peak condition.

Some of the things I have heard from greenhouse growers include:

• "I've added compost (or worm tea) but my plants don't look good and are growing slowly."
• "I've put in lots of manure, but the strawberry leaves seem to be curling and the tips are brown."
• "All my plant's lower leaves are yellow and falling off."
• "The blossom ends of my tomato fruits are black. I have heard this is a shortage of calcium."
• "All of the leaves on my lemon tree are pale green"
• "I have a lot of bugs so I sprayed my plants, but they still don't look good."

Let's unpack some of these greenhouse questions:

Understanding the Role of Worm Tea in Plant Nutrition

Worm tea is a diluted version of worm pee. When you make your own worm compost or worm castings (just to make it confusing, they are also sometimes called vermi-compost or fermi-castings) you are turning the worm munched debris into two things: the castings, or dry parts, and the leachate, or pee that drops out from below the finished worm castings.

The castings are packed with fiber, a few nutrients and loads of organic matter to feed microbes and help your soil support plant growth. Castings are soil-modifying, add beneficial bacteria, beneficial bugs, and some nutrients too. Worm castings are an excellent product, but they are not

If you rely on worm pee or castings alone, you are putting fast-growing greenhouse plants on a severe diet. Plan to add extra supplements (aka fertilizer) now.

Composted Manure: Benefits and Potential Drawbacks

In most cases, compost made from manure can help plants grow. Like worm castings, though, animal composts are very low in nutrients. Unlike worm castings, manure compost like “steer manure” often come with a negative side effect, a chemical used to kill thistle and other weeds becomes embedded in the manure. Meanwhile, this herbicide causes odd twisting of leaves and distortion of flowers in affected plants in the pea, bean, tomato, potato and sunflower family. Yes, you can pay for a test to determine if

Clopyralid is present in your bags of compost, but will you? Manufacturers rarely do. 

And because it causes havoc in parts per billion (a few drops of chemical in an Olympic-sized swimming pool)  these tiny amounts of chemical can and do cause trouble and will last in the soil for up to 20 years. So instead of a lab test, if you are worried about chemicals in compost, try growing a plant in one of the affected groups. It will tell you right away if it is happy.

Other problems with composted manures are browning of leaf tips, signs of low nitrogen and very dry soil. Avoid the use of manure in your greenhouse wherever possible, adding them to your compost first and then - after they break down - adding them to your greenhouse pots and soil. And if the plants are small or unhappy, add extra fertilizer.

Decoding Yellowing Lower Leaves

Plants need nitrogen, loads of nitrogen. Nitrogen is a big part of the DNA in every cell, and it makes leaves green. As the first number on the fertilizer bag, nitrogen is important to keeping leaves green but occasionally sulfur also greens leaves. If your plants are pale, they may need both nitrogen and sulfur.  

Here is one of my plants before adding nitrogen and micronutrients to my soil.  Plant leaves are small and pale. 

After adding nitrogen and sulfur to my soil, the garden started to lose its yellowing and was thriving again.

When the lower leaves of plants start to yellow first, this is because nitrogen moves around in plants from the old and shaded lower leaves to the newer growth up top. Over time though, and with extended nitrogen deficiency, the leaves of the plants get smaller, the overall color of the leaves gets paler, and the plant leaf surface becomes dull. Lower leaves turning yellow are begging you to add fertilizer, any nitrogen containing fertilizer. This will stop the deficiency and you can work out the details later. 

Pale or small leaves also indicate a nitrogen deficiency, while overly large leaves mean too much nitrogen fertilizer. It is a balance. Often, the lower leaves turn yellow first as plants move the very mobile nitrogen up to a plant’s growing points.  On my two potted zucchini plants I see the plant in the smaller pot needs more nitrogen than the one in a bigger pot. It is usually better to keep plants evenly moist and fertilized and this is harder to do in small pots. Meanwhile, the wet/dry cycle can trigger its own problems such as calcium deficiency.

Calcium Deficiency and Blossom End Rot in Greenhouse Tomatoes

When the bottom part of a tomato is black and flat, this is often called “blossom end rot.”  Calcium deficiency is the usual finger pointed for this problem and specialized tomato fertilizers can offer a “cure”. But instead of buying special fertilizers for everything I simply buy a fertilizer with micronutrients including calcium, zinc and others. 

More importantly, I look to find a way to keep greenhouse plants evenly watered because even with calcium added, some tomatoes show signs of blossom end rot when the soils go through dry and wet cycles. This can happen when you take off for a weekend or longer, as I did. Even with the best house sitter, perfectly moist soil is hard to manage all the time.  It is easier to get a pot with a water reservoir, called a SIP pot, than find a talented gardener to help in times of need. 

SIP pots are self-irrigated pots and they have a water reservoir in the base. Either homemade or purchased, the beauty of these pots is the consistent water supply. I love to encourage everyone to try Sub-Irrigated-Pots (SIP) that are now widely available. BC Greenhouse Builders even sells beautiful sub-irrigation self-watering garden beds that fit perfectly into their greenhouses.

PS - I’ve noticed my plants in SIP’s* are better than my soil-grown plants. Judging from emails and feedback I receive from other greenhouse growers I suspect this is because the moisture and fertilizer in SIPs are more slowly and evenly available to plants than those grown in greenhouse beds.

Pale Yellow Lemon Leaves: Nutrient Deficiencies and Insect Problems

When lemon leaves are all pale instead of just the lower leaves, you might have a nitrogen deficiency as described above or an added sulfur deficiency. But when I had a lemon tree dropped off recently because a gardener was frustrated with her pale sticky leaves, I discovered her real problem. I see scale, an insect problem, on the stems of the plant. With a strong stream of water, I quickly hosed off the plant. This removes some of the scale and the crawling stage of the insect. Then I prune the plant back heavily to bring it back into a small tree shape, and finally I fertilize it with a product that contains micronutrients. Just for good measure, and because it is summer and the plant is not blooming, I also add a specialty blooming fertilizer in a fine mist to the leaves. 

The leaves are also yellow between green veins, a condition called interveinal chlorosis, another micro-nutrient deficiency. High pH waters make it harder for plants to supply micro-nutrients from the soil. Because it is so hard to change your water or soil pH, try buying a fertilizer that includes chelated micronutrients like manganese, iron, calcium and sulfur they won’t be cancelled out by a high pH. Different fertilizers include different micronutrients, so read the fine print. Also, a special additive from the deep seas called kelp, is sometimes added to fertilizer and, trust me, it makes everything work better and gets plant cells vibrant again.

Pale lemon leaves might mean fertilizer trouble but as I discovered with the plant left on my doorstep, it might also mean an insect problem. Taking a closer look is what gardeners do.

PS - If the plant is small enough to move, take it outside for the summer. Naturally occurring insects like wasps are drawn to the sticky scale pee weeping onto affected leaves and wasps will quickly dispatch insect problems in combination with water streaming and fertilizer.

Avoid Sprays: Alternatives for Dealing with Insects Issues in the Garden

I remember when my neighbor popped over to let me know not to worry. She had just sprayed insecticide on her birch tree and all the little black bugs were now dead. When I went out to take a closer look, I confirmed her statement: there were millions of dead little black bugs. The trouble is, the dead bugs were immature lady beetles, a bug that was eating all the aphids on her stressed dry tree. 

Spraying plants kills both the good bugs and the bad bugs. Even products that seem simple like soap and water, mineral oil, or Neem oil will suffocate those hard-working beneficial bugs we are trying to draw in. So, instead of spraying with products, I encourage a vigorous stream of water on plants such as lemons because they tend to get spider mites, scales and other insect problems when their fertility levels fall off, when the soil dries out too much, or if the growing conditions make them stressed or weak. Just say no to sprays and start washing plants and attracting beneficial bugs when you can.

Achieving Peak Health: Observing and Nurturing Your Greenhouse Plants

A greenhouse is such a fantastic growing space we forget how fast plants grow in response to the near-perfect conditions. And the speedy growth means nutrients are being used quickly. To spot deficiencies start taking more photos, sitting in your greenhouse when you are resting, and watching for leaf size as well as leaf color changes over time. Plants will show us their true colors and nutrient deficiencies if we take the time to sit and watch them grow. In turn, we benefit from the Biophilia effect of being around nature. After taking a closer look, act by fertilizing or washing plants so you can grow peak condition plants.

How do you know when a plant is in “peak condition” and has everything it needs? Well fertilized plants will use the extra nutrients they have to build up a waxy layer on leaves. This waxy layer reflects light and makes plant leaves shiny. Whether I am buying spinach off-season for a special salad or growing lemons for myself I aim for leaves that are shiny. Shiny leaves are your sign that your plants, or the plant parts you buy, are in peak health. And peak health in plants means peak health in the people who eat those plants. Oh, and by the way, fake shiny plants sometimes have waxy coatings sprayed on leaves or fruit like apples and oranges. Stay away from artificial coatings that make plants shine and work on adding the fertilizers your plants need to make them glisten naturally.

Fertilizer will stress-proof plants, make the leaves shiny and the plants stronger.  Even if you add the wrong amount or wrong type of fertilizer at first, you will refine your choices as you try out different things. Organic, water soluble and fruit inducing? Slow release and long-lasting? Kelp or worm pee or compost? These choices and others are available. If your plants need a little help, build them back up for better harvests this fall.