Sweet Frost: Bitter Bolt
Have you ever noticed that your home grown leafy greens get more bitter as the plant grows on? Or that your broccoli is sweeter than you were expecting? Many factors, beyond type and variety of produce, influence how bitter or sweet a plant is, and flavor can change depending on the maturity of the plant and environmental conditions. Many plants will create plant chemicals in response to stress, including too hot or too cold temperatures and insect or animal browsing among other stressors.
Flowering can happen prematurely, i.e. bolting, from inclimate weather. When cool weather crops experience prolonged warm weather (75 and above), and warm weather crops experience prolonged cool weather (55 and below), the plant pushes to finish it’s life cycle by flowering and producing seed as fast as they can. When plants flower to produce seed, the plant’s physiology changes. Sugars, naturally created by the plant, accumulate in the flowers and eventually help to form the nutrient rich seed. These sugars have to come from somewhere and are often co-opted from other parts of the plants such as the roots and leaves, leaving these parts less sweet and more bitter in flavor.
Bitter Defense Against Bugs
Plants possess a variety of defenses against insects. Certain forms of bug defense are ever present like structural defenses such as spines and thorns, naturally occurring chemicals, and can even occur in the form of attracting natural enemies of the target pests. Some forms of bug defense happen in response to insect attack, which can make the plant in question toxic to insects and unpleasant to the human palate. Plants can tell when bugs are munching on them, as opposed to mechanical damage such as harvesting and weather damage, and respond accordingly producing secondary metabolites (compounds not directly involved with growth and development for the plant) such as glucosinolates, tannins, flavonoids, and increased calcium levels, all of which can taste bitter to humans.
Sweeter from Frosts
As Winter approaches and nights get colder and longer, many plants will begin to enact small changes on the cellular level to be able to overwinter (live and grow through Winter). During this “hardening off” process, the plant will slowly make their cells walls thicker and thicker. Without this, the first frost for your plants would be comparable to freezing and then defrosting produce; the cell walls burst as the water freezes and when it defrosts your fruit or veggie is limp and mushy. Gross!
When cool weather crops are planted with time to harden off (planted 4-6 weeks before the first frost), their thickened cell walls are better equipped to withstand when water within the cells freeze, without bursting. Progressively cooler weather also signals overwintering plants to create more sugars which they can metabolize and growth stays low and compact, which helps the plant to stay “warm”. Many of our cool weather crops such as turnips, beets, and carrots are naturally higher in their sugar to starch ratio, by no coincidence. This high sugar content also helps prevent them from freezing by lowering the cell’s freezing point. All of these things can contribute to the sweetness (or lack of) in your cool weather crops.
Although bitter flavors can be off putting to many, especially younger children and picky eaters, bitter flavors can be very healthful. Bitter flavors can be cut or masked by cooking, as opposed to eating raw, and by incorporating equally strong flavors such as acids like citrus juice or vinegars, garlic, and even sweet and spicy components.
The best way to avoid unusually bitter plants (although sometimes it can be inevitable), is to plant season appropriate plants while taking into account daytime and nighttime temperatures. This means you should be growing predominantly cool weather crops for Winter and Spring and warm weather crops for Summer and Fall. Reduce incidences of stress by raising your umbrellas on hotter days, keeping your plants mulched to reduce temperature swings, keeping your reservoir filled, and checking regularly for pests.
In the bittersweet end, incorporate some bitter greens and veggies into your garden and meals; your body will thank you!