Pollination is the movement of pollen from male to female flower parts of sexually reproducing plants. It is often accomplished by wind and insects and results in the development of some type of fruit containing seeds for the species’ continuation. Farmers and gardeners in the mid-Atlantic are finding that high day and evening temperatures can cause vegetable plants to drop flowers and small fruits or produce deformed and under-sized fruits. This problem has been observed in crops like bean, tomato, and pepper (mostly self-fertile; individual flowers can pollinate themselves), and in crops like squash and pumpkin (require cross-pollination between flowers).
How do high temperatures affect pollination?
All fruiting plants have an optimal temperature range for the pollination/fertilization process. High temperatures can reduce pollen production, prevent anthers from releasing pollen, kill pollen outright, and interfere with the pollen tubes that serve as conduits for uniting sperm cells and eggs (fertilization) inside undeveloped seeds (ovules). High temperatures can even injure flowers before they open. Night temperatures are increasing at a faster rate than day temperatures as a result of climate change, and seem to be most responsible for these pollination problems.
Soils, plants, and animals are highly interdependent. Soils support and feed microbes and plants which feed animals. Dead plants and soil critters replenish the soils’ organic matter and nutrient supply, completing the cycle. We know that healthy soils produce healthy plants. Many experts believe that improving soil health is the most important thing we can do to make our farms and gardens more climate-resilient.
Why are soils so important in dealing with climate change?
They store huge amounts of carbon in the form of carbon dioxide (CO2) and organic matter, all of the living, dead, and decomposing plants, microbes, and animals that live in soil. Carbon dioxide is the primary greenhouse gas that is warming the planet. Deforestation, the removal of wetlands and peatlands, and soil tillage cause the release of huge amounts of CO2. Warmer temperatures cause more rapid organic matter decomposition and turnover, especially if soils are tilled and uncovered.
Climate change is causing mid-Atlantic weather to be warmer and wetter with more extreme weather events, including periodic drought. This increases the risk of soil erosion and nutrient run-off from intense rainfall, and the risk of plant stress from excessively wet or dry soils.
Every time we plant a seed or baby plant in our vegetable garden we are hoping for the best outcome- a healthy crop and big harvest. Gardening success comes from learning about the needs of our crops and doing all we can to meet those needs. Climate change is causing us to think a little more deeply and holistically about those plant needs and our gardening practices.
In addition to making sure that plants have enough space, water, and healthy soil, we can alter how and where we plant our crops (“comfy places”) to help them adapt to increasing summer temperatures. We can also consider ways to expand or shift our food garden spots (“new spaces”) to better manage growing conditions and produce more food.
Sphagnum peat moss is valuable in horticulture because its fibrous structure helps it retain a lot of water and air while draining excess water. This has made peat a primary ingredient of soilless growing media (potting mix) around the world. These stable, light-weight, and porous products have been filling the benches, flats, and containers of greenhouse and nursery operators and flower and vegetable growers for decades. You’d be hard-pressed to find a gardener who has not benefited from soilless potting mixes for starting and growing plants, inside and outside.
What’s the problem with peat?
Peat is an organic substance formed from mosses, reeds, and sedges that accumulates and decomposes very slowly in waterlogged soils (bogs). Peatlands hold 30% of the earth’s soil carbon and occur mostly in cold, temperate regions. “Peat moss” used in horticulture typically refers to mosses in the Sphagnum genus.
The problem with peat is three-fold: stripping off peat from peatlands disturbs complex ecosystems; excavation releases enormous amounts of CO2, a major greenhouse gas driving climate change; and demand for peat-based soilless media is growing.
For decades, there have been calls to conserve the U.K.’s dwindling peatlands. Timelines are in place for soon phasing out peat as a growing media for gardeners and commercial growers. Most sphagnum peat is from Canada and there are no indications that Canada, with its vast peat reserves, will follow suit. But public demand for peat-free alternatives will drive the industry to develop new products.
Reducing the use of peat in horticulture will mitigate climate change and increase reliance on local materials as peat substitutes.
Our food-growing spaces allow us to grow healthy produce, connect with Nature, and hopefully save money. They are also a solid response to climate change and COVID.
My blog articles this year will be about climate-resilient food gardening. Each month I’ll address one or more aspects of how climate change is affecting our food gardens and changes we can make to reduce global warming and ensure a future of healthy harvests.
HGIC has a new Climate-Resilient Gardening section (thanks to Christa Carignan!) where you’ll find more information on these topics. We plan to continually update content and add new pages. And please check out the University of Maryland Extension’s new Healthy Garden, Healthy You project that connects food gardening and human health.
This first installment includes an overview of how our mid-Atlantic climate is changing and a look at heat-tolerant crops and cultivars. Future articles will explore low-dig soil prep, composting food scraps, peat alternatives, heat stress in plants, reducing plastics, and “hardening” our garden spaces.
Resiliency is mentioned a lot with respect to climate change. A climate-resilient garden can both withstand and recover from warmer, more extreme weather. Resiliency can also mean transforming how we grow food by creating and sharing a community knowledgebase of new ideas and techniques.
Farmers and gardeners learn much by daily tending soils and plants. But the winter “off-season” affords us time to dig deeper into topics of interest and learn from co-cultivators and experts in the field. I spent some time in a variety of grower meetings, conferences, and webinars in January and February where research findings were shared. Many gardeners are interested in growing blueberry so in this article I’ll share tips for success including some insights I picked up from presentations by Oregon blueberry researchers David Bryla, Ph.D. (USDA) and Bernadine Strik, Ph.D. (Oregon State U.)
Highbush blueberry plants evolved to grow in low pH, high organic matter sandy soils with high water tables. These soils contain more ammonium nitrogen than nitrate nitrogen, hence blueberry’s preference for the ammonium form of plant-available nitrogen. The shallow, fibrous root system grows almost entirely in the top 12 inches of soil. Most of the roots are very fine, the width of a human hair, and can’t penetrate or thrive in clayey, compacted soils. The key to success is create garden conditions that mimic those in blueberry’s natural environment.
Blueberry thrives in well-drained, porous soils, high in organic matter (4% – 20%). The soil pH should be in the 4.5-5.5 range.
Soil preparation starts in fall
Begin by testing the soil in the late summer or fall prior to spring planting. For gardeners, soil testing labs provide the most accurate pH measurement of your soil, as well as baseline information on organic matter and nutrient levels. pH probes sold to gardeners are generally inaccurate and pH color kits using litmus paper are only accurate to ½ of a pH unit (5.5, 6.0, 6.5, 7.0, etc.
Add organic matter
The top 12 inches of soil should be one-third to one-half organic matter by volume. Peat moss (3.0-4.5 pH), plant-based compost (7.0-7.5 pH), and lightweight potting soil, a.k.a. soilless growing media (5.5-6.5 pH) are the materials most often mixed into the soil. Research has shown that adding compost (especially animal manure compost) can increase soil pH.
Some Oregon growers incorporate 2-3 inches of aged softwood sawdust into topsoil prior to planting. The benefit is that sawdust has a low pH, decomposes slowly, and increases organic matter levels. For Maryland gardeners, large amounts of sawdust are difficult to come by, but bark fines are readily available. You would need to apply 1.0 lb. of additional ammonium sulfate (21-0-0) per 100 sq. ft. nitrogen for the soil microbes that slowly decompose the bark fines.
Lower soil pH
Elemental sulfur is applied (based on soil test results) in the fall prior to spring planting, and incorporated to a 6-8 inch depth.
Pelletized and prilled forms of sulfur are easier to apply than powdered sulfur but take longer to lower soil pH.
An oxidation process, driven by special soil bacteria, converts the sulfur to sulfuric acid, releasing hydrogen ions that lower soil pH. The bacteria are most active in warm, moist soils. The process takes 6-12 months. Iron sulfate can also be used to lower soil pH but 6 times as much is required, increasing the cost.
Re-test soil pH to monitor pH levels and apply sulfur as needed to maintain the 4.5-5.5 range.
For container blueberry plants, mix 3 TBS. of sulfur into the top few inches of growing media, for a 15-gallon container, to reduce the pH by one unit (e.g. from 7.0 to 6.0).
Ammonium sulfate fertilizer is recommended because it supplies nitrogen in the ammonium form and helps acidify the soil.
Fertilize at full bloom and again three weeks later.
Urea is another good nitrogen source, recommended when soil pH is below 5.0 because it is only one-half as acidifying as ammonium sulfate. The nitrogen in urea is converted to ammonia and then to ammonium.
Oregon research studies show that feathermeal (12-1-0.5) and soluble fish fertilizers (4-1-5) work well in organic blueberry production. Organic growers prefer to inject fertilizers into irrigation water, known as “fertigation.” Another interesting finding was that there were no significant yield differences between the lowest (20 lbs./acre) and highest (240 lbs./acre) nitrogen fertilization rates.
Organic matter and organic fertilizers release ammonium ions with relatively little oxidized to the nitrate form as long as soil pH is in the 4.5-5.5 range. When soil pH is >6.0 most of the nitrogen from decomposing organic matter will be converted to the nitrate form with negative effects on plant growth.
Oregon research indicates that organic acids (humic and fulvic) applied in liquid form, increase blueberry root growth while lowering soil pH.
Blueberry root systems need to be kept moist. Plants can tolerate hot weather but not drought. Water your blueberry bed thoroughly and consistently when rainfall is lacking. Soaker hoses and drip irrigation work well.
Blueberry grows and produces best when the pH of irrigation water is <7.0. Commercial growers often acidify irrigation water to maintain low soil pH. The pH of municipal water in our region is typically 7.5-7.8 and has a high salts and bicarbonate content. Just be aware that your irrigation water can drive up soil pH.
Blueberry roots cannot compete very well with weeds for nutrients and water. Mulch is essential to keep soil cool, improve water infiltration, conserve soil moisture, reduce weeds, and increase organic matter.
Use aged wood chips (never fresh), shredded bark, pine needles, or sawdust as a mulch. These materials are low in pH (4.5-5.2) and salts, and decompose slowly.
Interestingly, a recommended growing system in Oregon uses strips of heavy-duty weed barrier to cover beds after they have been mulched to further reduce weed growth and moisture loss.
A well-planned and maintained blueberry bed can produce well for 20+ years. Start yours in 2021!
So let’s get this season started as soon as possible with cool-season crops. These are the plants adapted to grow best with cooler daytime temperatures, 65⁰ F-75⁰ F, compared to warm-season crops like tomato and chile pepper. They are planted in Maryland from early March through mid-May and again from July through September. Thinking about these crops in January gives us time to plan and prepare for success!
Some cool-season crops are hardy, very frost-tolerant, like pea, spinach, and onion. Others are semi-hardy, more easily injured by cold temperatures, like beet, carrot, and lettuce (although this depends a lot on cultivar, stage of development, and growing conditions). Cool-season crops germinate at lower soil temperatures (40⁰F-45⁰F) than warm-season crops like cucumber and squash (55⁰F-60⁰F).
Some cool-season crops need both cool and warm temperatures. Onion and garlic need cool temperatures for rapid leaf growth, and warm temperatures for bulb enlargement. Leafy greens grow well in spring and fall while broccoli and cauliflower tend to produce better yields and higher quality heads in fall. That’s because leaf and root growth is favored by warm temperatures and head development is best with cooler weather.
We’re mostly interested in eating the leaves, stems, and storage roots of cool-season crops. Cool temperatures cause these crops to produce more sugars which makes them more cold-tolerant and better tasting. Rising temperatures can reduce crop quality, causing bitter or off-flavors, and force plants to send up flower stalks and produce seeds (bolting). Increasing daylength may also induce bolting in lettuce, spinach, radish, potato, and carrot.
A few cool-season crops are perennials (rhubarb, horseradish, and asparagus) but most are grown as annuals, even though many are biennials.
Most commonly-grown cool-season annual crops by plant family:
Bean family (Fabaceae)- garden pea, fava bean
Cabbage family (Brassicaceae)- radish, broccoli, cabbage, Brussels sprouts, cauliflower, kale, mustard, turnip, collard, kohlrabi, Asian greens, Chinese cabbage, rutabaga, arugula
Garlic family (Amaryllidaceae)- garlic, leek, onion, shallot,
Beet family (Amaranthaceae)- spinach, beet, Swiss chard
Carrot family (Apiaceae)- carrot, parsnip, celery, celeriac
Lettuce family (Asteraceae)- lettuce, endive, escarole, radicchio
Seeds or transplants– most gardeners plant cabbage, broccoli, cauliflower, and Brussels sprout transplants but sow seeds directly in the garden for all other cool-season crops. This year, try growing or buying transplants instead. Even peas can be started indoors under fluorescent or LED lights.
Growing lettuce, kale, pea, arugula, etc. for just 2-3 weeks indoors will give you a head start on the growing season and eliminate the need for thinning excess plants (tedious!). Transplants are also less likely than seeds to be washed away in a storm. Even carrot and beet can be transplanted as long as the tap root doesn’t hit the bottom of the container while growing indoors.
Soil preparation and nutrients– it’s possible to plant and grow warm-season crops, like tomato and squash, in cloddy, clayey soil. The same is not true for most cool-season crops. Whether you plant seeds or transplants the topsoil should be loose, well-aerated, and high in organic matter. Heavy, cloddy soil slows seed germination and restricts root growth.
Preparing a bed in spring requires minimal tillage- cutting winter annual weeds at the soil with a weeding tool, fluffing the soil with a garden fork, and raking the soil smooth. If a cover crop is in place, cut it to the ground with a string trimmer, cover the area with a tarp or weed barrier for 2-3 weeks, remove it and plant through the cover crop residues.
The majority of cool-season crops have a medium to high requirement for nutrients. Fertilize seedlings and transplants in spring with a soluble fertilizer to get them off to fast, strong start. Nutrient release from soil organic matter is low at this time and root systems are too small to adequately mine the soil for nutrients may be inadequate. Fertilize fall crops once they are established.
If the soil is not suitable, plant in containers filled with a mixture of compost and soilless growing media (lightweight “potting soil”).
Protection– climate change is giving us a longer fall season allowing big harvests of cool-season crops into December. But climate change is also making spring weather even more unpredictable than “normal.” The average last frost date is earlier but seedlings and transplants are subject to wide temperature swings, excessive rainfall, and late freezes (as we saw in 2020).
Floating row covers and cold frames provide a buffer against unfavorable and rapidly changing weather conditions. They allow us to extend the gardening season so that we can plant earlier, harvest latter, exclude insect pests, and increase yields. Protective covers, whether, glass, clear plastic, or polyester fabrics, increase the air temperature around plants and reduce damage from wind and driving rainfall.
Timing– planting calendars are helpful but planting decisions should also be based on the 7-day forecast. Transplants give you some added flexibility as they can be set out earlier or held back depending on conditions. The key is to have all the pieces in place- seeds or transplants, prepared soil, protection, and fertilizer- when conditions are right for planting. Some years you may find yourself planting with a headlamp or flashlight!
Future articles will focus on planting and caring for specific cool-season and warm-season crops, and how to extend the gardening season and adapt to climate change.