Growing Degree Days (GDD) is a heuristic used by horticulturists and gardeners to predict the date that a flower will bloom or a crop reach maturity. Below a certain temperature, plants grow very slowly if at all and above a certain temperature, their growth is "maxed out" but between those two ranges, the relationship between plant growth and temperature is close to a straight line. Growing degree days take aspects of local weather into account and allow gardeners to predict (or, in greenhouses, even to control) the plants’ pace toward maturity. Of course, this assumes no extreme conditions such as drought or disease.
BeeCulture published an excellent primer on GDD in Feb 2002. GDD is an effective way to predict what will be blooming in your immediate area and can be invaluable in deciding when to add supers to your colonies.
GDD is calculated by taking sum of the average of the high and low temperature each day compared to a baseline.(Tmax + Tmin)/2 - Tbase
The baseline varies by plant type but for many temperate plants and even insects, a good baseline is 50° F (10°C). For example, a day with a high of 70°F and a low of 54°F would contribute 12 GDDs. (70 + 54)/2 - 50 = 12 Cumulative GDD is generally measured from the winter low.
Any temperature below the baseline is set to the baseline before calculating the average. For example, a day with a high of 56°F and a low of 44°F would still contribute three GDDs. Likewise, the maximum temperature is usually capped at 86°F (30°C) because most temperate plants and insects do not grow any faster above that temperature.
Bloom duration is harder to measure. In some species, it too is a function of GDD. In others, bloom duration is a function solely of time. And it's always subject to weather. No matter what GDD says, the apple bloom is over after the spring storm rips all the petals off the trees.
Bee Friendly pest control for gardens
Since 1998, scientists, conservationists, and farmers have noticed an alarming trend. European honeybee populations are declining at rapid rates. Researchers believe several factors are at play here: viruses spread through the colonies, loss of habitat, migratory habits, and increased pesticide use.
One class of pesticides, in particular, has been implicated in the decline of bee populations. The European Council recently voted to ban this group of pesticides, neonicotinoids, in an effort to restore bee colonies.
Over 70 percent of the world’s crops are at least partially pollinated by bees. Without these pollinators, we would face a substantial loss of crop diversity. In your own garden, a loss of pollinators means no more zucchini, pumpkins, summer squash, apples, berries, or melons. Some crops, such as carrots and onions, rely on bees to produce seed, rather than edible plant parts.
The need for safer pest control measures is apparent, but you might be wondering how to control pests without harming bees. Read on for a roundup of ideas.
Organic Pest Control
Many pesticides are labeled as safe for organic use. These products are usually derived from plants or other natural materials and they might break down faster in the soil than synthetic products. However, just because a product is labeled “organic,” doesn’t mean it won’t harm bees. Below is a list of common organic pesticides, fungicides, and herbicides, and their toxicity to bees, according to the Xerces Society. When using a pesticide, always opt for the least toxic products.
How you apply pesticides can also make a difference in their toxicity to bees. Pesticides kill bees in several ways. First, bees absorb the chemicals through their exoskeletons when they’re exposed to pesticides in the air. Pesticides can contaminate pollen, nectar, and even dew. When bees come in contact with these substances, they can be killed. Finally, bees sometimes take contaminated pollen and nectar back to the hive, where it harms the other bees.
If you must apply pesticides, apply them only to the affected plants, and preferably when the plants aren’t in bloom. Apply them late in the evening when bees aren’t active. Spray pesticides during dry conditions, if possible, since dew can retain the toxins.
Whenever possible, focus your efforts on prevention strategies, rather than resorting to pesticides to eliminate problems. Organic gardeners take a holistic approach, knowing that no one strategy can combat every pest. When used in combination, though, the following techniques can minimize the pests in your garden.
Floating Row Covers. Floating row covers are lightweight agricultural fabrics that allow water and sun to permeate while keeping out the bad guys. Install them at planting time and secure them with rocks or pins. Allow enough slack for the fabric to expand as the plants grow. Floating row covers won’t eliminate pests that live in the soil, but they will keep out beetles, flea beetles, and many other pests. Additionally, floating row covers warm the soil and keep it moist so seeds germinate faster. They also encourage faster growth in young seedlings. Remove row covers when summer heat arrives or when plants start to produce flowers so bees can pollinate them.
Hand Picking. This strategy might not appeal to squeamish types, but if you were the sort of kid who loved bugs, this might be right up your alley. Simply stroll through your garden every morning or evening and pick off caterpillars, beetles, snails, and slugs. Drop the pests in a bucket of soapy water. Obviously, this technique won’t work for flea beetles or small pests, but it’s a fast and simple way to control most of the pests in your garden.
Traps. Veteran gardeners seem to collect creative methods for trapping insects. Wrap a paper collar around tender young seedlings. Push the collar down into the soil at least 3 inches, allowing another 3 inches to remain above ground. Collars are effective at deterring cutworms and other soil-dwelling pests. Some gardeners lay crumpled aluminum foil under plants to keep cutworms, slugs, and snails out. Others place shallow bowls of beer or soapy water in the garden to attract and drown insects. Traps work with varying effectiveness, but are worth a try. Don’t leave anything toxic, such as mothballs, in the garden where children and animals might get it.
Crop Rotation. One of the simplest ways to control pests and diseases is by rotating your crops from year to year. Some insects are generalists, nibbling their way through the garden like it’s a buffet. Most, though, prefer some plants over others. Cucumber beetles prefer cucurbits, tomato hornworms love tomatoes. Move the crops around and you’ll confuse the pests a bit. Combine crop rotation with inter-planting and you double your odds of success. Inter-planting is the technique of planting two crops together to repel, confuse or deter pests. For example, onions and garlic seem to repel many pests. Plant them among your more vulnerable plants for added protection.
Invite Beneficial Insects To The Garden. Not every insect is a pest. In fact, some insects can help your garden by feeding on the bad guys. Ladybugs, lacewings, and praying mantises are three insects worth keeping around. Plant flowering herbs in your garden, such as dill, angelica, cilantro, and basil. Beneficial insects often lay their eggs on these plants and their larva feed on them.
Tidy Up. Keeping a tidy garden has more than aesthetic benefits. A clean garden harbors fewer pests. Weeds and grasses growing near your garden can harbor aphids and other pests that not only feed on plants, but carry diseases, such as aster yellows. Overripe produce is sure to attract pests as it rots. Always harvest produce as it ripens and freeze, compost, or give away what you can’t use immediately.
Finally, sometimes the simplest solution is to take a live and let live approach. A few damaged leaves are rarely fatal to plants and hardly worth the risk of using pesticides. If a specific crop is particularly troublesome, perhaps it’s time to drop it from your vegetable garden inventory. A few simple changes this summer can mean a healthier garden for both you and the bees.