Introduction: Growing Gardens Year-Round

As an urban gardener, I dream of having fresh vegetables and herbs to harvest year-round. While the main growing season winds down as winter approaches, there are many ways to extend the harvest with a little creativity and some innovative heating techniques.

With the right solutions in place, I can keep my urban garden productive during the colder months. This allows me to save money at the grocery store and enjoy garden-fresh produce when other local options are limited. A longer growing season also leads to more crop rotations and higher total yields from my small space.

In this article, I’ll share the most effective methods I’ve found for heating my urban garden to prolong the harvest. We’ll look at passive solar techniques, row covers, greenhouses, and more. I’ll also give specific tips on Which crops grow best in cooler weather.

Let’s dive in and explore all the ways you can keep growing delicious, nutritious food straight from your backyard all year long!

The Benefits of an Extended Growing Season

Having a longer harvest period provides many advantages for the intrepid urban gardener:

• Higher total yields – More time for growing means more crops can be cultivated over the course of a year, leading to greater quantities of fruits and vegetables.
• Bigger variety – Out-of-season crops allow for more diversity in your backyard harvest.
• Better use of space – Multiple successive plantings let you make the most of your limited square footage.
• Year-round access to fresh produce – Avoid grocery store produce that may lack flavor and nutrition.
• Lower food costs – Growing your own food saves money, which really adds up when eating from the garden for more months.
• More crop rotations – Rotating plant families helps control diseases and maintain soil health.
• Greater self-sufficiency – Rely less on commercial supply chains and become more food independent.

With so many benefits, investing time and effort into extending the growing season makes sense for any veggie-loving urbanite. Let’s look at how it works.

Understanding Your Climate Zone and Growing Season

The length of your region’s natural growing season depends on climate factors like:

• First and last frost dates – The periods when temperatures dip below freezing in spring and fall.
• Summer high temperatures – Heat loving plants thrive when average highs are above 75°F.
• Winter lows – Consistently frigid winter lows below 20°F limit overwintering options.
• Sunlight levels – Plants need adequate sunlight, which varies by season and latitude.

To understand your specific urban microclimate, track local conditions over several years. Online tools like the National Gardening Association’s Zip Code Zone Finder can estimate your zone details.

Knowing my climate zone and average seasonal patterns is essential context for choosing effective season extending techniques. Now let’s look at proactive steps to add weeks or months to the harvest.

Start Seedlings Indoors for an Early Start

An easy way to get a head start on the growing season is starting seeds indoors up to two months before your last expected frost date.

I start quick-growing salad greens and brassicas indoors under fluorescent grow lights as early as 6 weeks before transplanting. Heat-loving tomatoes and peppers go in 4 weeks prior. This gives me mature seedlings ready to plant out ASAP to maximize yield potential.

If you lack sufficient indoor space for seeds trays, consider grow lights designed just for seed starting. They take up minimal room and provide the light intensity seedlings need.

With my indoor seed starting setup, I’m able to transplant crops outside weeks ahead of nature’s schedule. This fast tracks the harvest by a month or more.

Use Cold Frames and Row Covers to Protect Plants

Once my seedlings are established in the garden, I extend the fall season further using protective devices like cold frames and floating row covers:

• Cold frames are bottomless boxes with transparent lids that sit over plants, trapping sun’s warmth. I build them from old windows and scrap lumber.
• Row covers made of porous fabric let sunlight and water reach plants while retaining heat. I secure them with metal hoops.

On cold nights, these covers prevent frost from ending the harvest prematurely. They also shield plants from excessive wind and rain damage. Plus bees can still access flowers for pollination.

With a combination of cold frames and floating row covers deployed strategically, I’m able to delay first frost by weeks and keep many crops alive well into winter.

Try a Hoop House or Greenhouse for Maximum Heat

For the most effective season extension, I rely on passively heated structures called hoop houses and greenhouses. These capture, retain, and maximize the sun’s warmth for plant growth.

Hoop houses consist of large hoops made from flexible metal or PVC piping and covered with greenhouse plastic sheeting. The basic versions are inexpensive and easy to DIY. They offer great bang for buck when it comes to heat gain.

For the ultimate in warmth and protection, I installed a permanent greenhouse made from glass or polycarbonate panels in a strong aluminum frame. While pricey, a greenhouse allows me to grow crops straight through winter with backup heating systems.

Greenhouses also provide shelter from excessive wind, rain, and pests while maintaining ideal humidity levels. They take season extension to the next level in my urban garden.

Passive Solar Heating for Greenhouses and Hoop Houses

The most cost effective way to heat a greenhouse or hoop house is tapping into the free abundant energy from the sun, also known as passive solar heating.

Here are effective techniques I use:

• Face greenhouse south for maximum winter sun capture and angle glazing at 60-75 degrees.
• Use thermally massive materials like stone, concrete and water barrels inside to absorb and slowly radiate heat.
• Insulate northern wall and foundation to prevent heat loss.
• Seal any air leaks thoroughly.
• Whitewash the interior in summer to limit overheating and install vents for ventilation.
• Add passive ventilation tubes along the roofline to let hot air escape.

With good passive solar design, my greenhouse quickly climbs into the 50s and 60s on sunny winter days – perfect for cool weather crops!

Active Heating Systems for Greenhouses

When passive solar gain isn’t enough, I supplement with active heating systems in my greenhouse. These require electricity or fuel to generate warmth.

My go-to options:

• Electric heaters – Affordable and easy to install, I use electric resistance heaters as backup on cloudy periods.
• Gas heaters – Propane gas heaters offer more power and efficiency than electric, but need proper ventilation.
• Hydronic heating – This system uses a boiler to heat water and circulate it through pipes under beds.
• Compost heating – Pipes buried in an internal compost pile distribute gentle warmth.

With an energy efficient combo of solar gain and active backup, I’m able to maintain my greenhouse in the 40s or warmer throughout winter.

Next let’s look at specific types of electric, gas and hydronic heating systems.

Electric and Gas Greenhouse Heaters

To provide a boost on sunless winter days, I rely on electric and gas heaters designed for greenhouse use.

Electric Heaters

Electric resistance heaters consist of coils that heat up when electrical current passes through. Pros:

• Inexpensive to buy and install.
• Thermostats maintain precise temperature.
• Safe for humid greenhouse environments.
• Portable units can move to where heat is needed.

The downsides are higher operating costs and lack of power in extreme cold. I use electric to maintain greenhouse warmth, not provide primary heating.

Gas Heaters

Propane gas heaters burn fuel for very efficient heating. Advantages are:

• Lower operating cost than electric.
• More heating power for cold snaps.
• Permanent units mount overhead and distribute warmth.
• Heated air rises and circulates naturally.

The main drawbacks are upfront cost, need for ventilation, and refilling propane tanks. Overall, propane makes an effective primary winter heat source.

Radiant Floor Heating and Hydronic Systems

One of my favorite greenhouse heating methods is hydronic radiant floor heating. This circulates hot water from a boiler through tubing under the floor or beds.

Benefits include:

• Even, gentle warmth from the ground up.
• Doesn’t affect greenhouse humidity or CO2 levels.
• Lower long-term cost than gas or electric heat.
• Doubles as a heat sink, storing warmth in concrete or water.

The boiler can run on gas, wood, solar thermal, or even just compost heat!

Installing the tubing system takes expertise, but the payoff is fantastic when sipping tea in the greenhouse on a winter day!

Solar Water Heating Systems

One way I obtain hot water for my greenhouse’s radiant floor heating is by using a solar thermal system.

This consists of solar collectors that heat either water or antifreeze solution as it passes through tubes in a glazed panel.

On sunny days, the hot fluid travels to an insulated storage tank before being circulated through the hydronic system as needed.

I maximize solar gain by:

• Facing collectors south at a 45 degree tilt.
• Using rooftop collectors to avoid shading.
• Insulating all pipes and the storage tank.
• Installing a pump controller and differential thermostat.

Combining solar water heating with a hydronic system allows me to tap into free sunshine for greenhouse warmth all winter!

Geothermal Heating and Cooling

The ultimate in efficient, renewable greenhouse heating is a geothermal ground source heat pump system.

This uses a network of underground pipes circulating water or coolant that either absorbs heat from, or dumps heat into, the surrounding soil.

A heat exchanger and heat pump optimize the process, providing warm water for heating and cool water for summer cooling.

Benefits of geothermal heating include:

• Extreme efficiency with COP of 3-5.
• Low greenhouse CO2 emissions.
• Provides both heating and air conditioning.
• 30%+ lower operating costs than alternatives.
• Tax credits and incentives available in many regions.

The main barriers are the high upfront installation cost and available outdoor space for the ground loop field. But for serious year-round growers, the investment in geothermal energy pays dividends for decades to come.

Compost and Manure for Internal Heat

Harnessing the innate heating power of decomposing organic matter is a time honored greenhouse heating method. As microbial activity breaks down compost and manure, it generates significant warmth.

There are two main approaches:

Compost piles

By housing thermophilic compost piles directly inside my greenhouse, I can distribute their natural heat in a couple ways:

• Burying hydronic heating pipes inside the compost to heat water.
• Placing heat exchanging ventilation pipes under the pile.

Hot boxes

This involves storing compost or manure in an insulated box within the greenhouse. Fans blow warmed air into the growing space. Hot boxes easily maintain temperatures in the 60s.

With a bit of tinkering, I’m able to leverage compost as a reliable backyard heating source for my greenhouse!

Thermal Mass in the Garden

Beyond structures like greenhouses, I use thermal mass principles to retain heat in my open garden beds.

Thermal mass refers to materials that absorb, store, and slowly release heat energy. This evens out daily temperature swings, keeping plants warmer at night.

Some examples I incorporate:

Water barrels

Filled rain barrels soak up daytime sun and radiate warmth after sunset. They also raise humidity.

Stone walls

Rock, brick and concrete wall segments along the beds soak up solar energy and release it back to plants.

Gravel mulch

A layer of pea gravel mulch holds daytime heat to minimize the drop after sundown.

Thermosyphon tubes

Using these passive heat transporting tubes, I can collect warm air from south-facing walls and distribute it to colder parts of the garden.

By optimizing thermal mass, I stretch my growing season weeks longer without any electricity or fuel required!

Heat Absorbing Materials Like Stone, Brick and Concrete

Certain building materials absorb and retain heat really well for radiating warmth to plants. By incorporating them strategically, I can soak up more free solar energy to extend the growing season.

Some prime thermal mass options include:

• Stone (granite, limestone) arranged around beds.
• Brick wall segments and pathways.
• Concrete blocks and slabs around the garden perimeter.
• Sand or gravel mulch over soil beds.
• Containers of water that absorb warmth.
• Decorative cement features like benches, steps and sculptures.
• Even black painted raised beds or barrels that absorb more daytime heat.

Taking advantage of these readily available heat banking materials makes reaching into November and starting in February that much more achievable in my urban garden!

South Facing Walls and Windows Maximize Solar Gain

To get the most out of the warming sunlight, I orient key garden elements to the southern exposure.

South facing walls and fences

Painting these black increases heat absorption. I also train fruit trees as espaliers against the walls to benefit.

Greenhouses and cold frames

South Facing Walls and Windows Maximize Solar Gain

Greenhouses and cold frames

Situating these heat capturing structures along the south maximizes solar gain during cooler months.

Windows

Strategic southern windows on my home, garage, and outbuildings allow more direct sunlight into the backyard, passively raising temperatures.

Ponds

Siting ponds and water features along the southern end of my lot allows the water to collect and radiate more heat.

Capitalizing on south facing building aspects prevents shade and optimizes every bit of available winter sunshine.

Windbreaks Help Retain Heat

One hidden culprit that can quickly sap precious heat from an urban garden is excessive wind exposure. Gusty breezes whisk away the warm boundary layer of air around plants.

By constructing windbreaks, I can protect my crops and conserve precious thermal energy.

Some effective windbreak options include:

• Walls & fences – Solid barriers block wind to create a calm microclimate.
• Hedges & trellises – Evergreen hedges and trellised vines provide living windbreaks.
• Groundcovers – Low growing groundcover plants prevent winds near soil level.
• Bales of hay – Stacked straw bales make temporary wind barriers.
• Reed fencing – Intact reed clumps filter wind while allowing airflow.

With thoughtful wind protection, I prevent excessive convective heat losses while extending the harvest season.

Insulate Raised Beds for Added Warmth

Raised garden beds already warm up earlier than at ground level. I get even more heating benefits by insulating the sides of my raised beds.

Some suitable insulating materials I use are:

• Bales of straw – Tucked around the beds, straw offers great insulation value.
• Leaves – Oak leaves piled around raised beds don’t break down too quickly.
• Bubble wrap – The plastic sheets trap air to prevent heat loss through the sides.
• Foam board – Rigid insulation panels come in various thicknesses.
• Newspaper – Surprisingly effective insulation when layered thickly.
• Cardboard – Corrugated cardboard folded around beds adds dead air space.

The insulation keeps the soil warmer at night, extending the growing period by weeks without any added energy input!

Use Cloches and Cold Frames Over Beds

Earlier I discussed using standalone cold frames and cloches to protect plants. I also maximize their effect by installing these directly over my raised beds as season extending techniques.

Some examples include:

• Bell jars or cloches over individual plants or rows.
• DIY cold frames attached atop the beds.
• Removable clear plastic covers draped over hoops.
• Straw bale walls packed around the bed edges.
• Low plastic tunnel covers over pairs of raised beds.

Having these modular covers available lets me quickly shield my crops as temperatures drop, keeping the fall harvest going strong.

Low Tunnels for Row Crop Protection

For protecting large areas like rows of vegetables and berries, I rely on low tunnel techniques:

• Wire hoops covered in greenhouse plastic sheeting.
• Flexible PVC pipes that create mini-greenhouses.
• Reinforced wire fencing covered with greenhouse plastic.
• Bamboo and shade cloth tents over crops.

I space hoops every few feet down crop rows and drape plastic sheeting over them, sealing the edges with soil.

Low tunnels allow good airflow while trapping warmth and protecting plants from wind damage. Quick to install and cheap to build, they buy me weeks of extra harvest.

Floating Row Covers for Flexible Heat Retention

An easy and effective way to prevent heat loss from raised beds or rows is using floating row covers.

These are made from spun polyester fabric that allows about 85% light transmission while retaining heat.

Benefits include:

• Lightweight material placed directly over plants.
• Provides 4-8 degrees of frost protection.
• Allows rainfall and irrigation to penetrate.
• Keeps pests away when edges are sealed.
• Can be lifted temporarily for access and ventilation.

I use short hoops to keep the material elevated and prevent contact with plants

Mulch for Soil Warming and Weed Suppression

Applying a layer of mulch over garden beds offers multiple benefits beyond retaining heat:

• Suppresses weed growth.
• Conserves soil moisture.
• Minimizes erosion.
• Keeps plant roots cooler in summer.
• Adds organic matter as it decomposes.

For season extension, I apply at least 2-4 inches of mulch after soils have frozen. The insulation this provides prevents rapid heat loss so soil stays warmer at night and thaws earlier in spring.

Thick mulch also helps prevent frost heave, which can push plants and seedlings out of the ground during freeze-thaw cycles.

Plastic Mulches vs. Organic Mulches

For maximum insulation and warmth, black plastic mulch is ideal. The black color absorves more solar radiation while the plastic prevents evaporation and traps heat against the soil.

But plastic must be removed each season. For sustainable mulching, I rely on loose organic materials like:

• Straw
• Leaves
• Pine needles
• Compost
• Shredded newspaper
• Cardboard
• Wood chips

Organic mulches don’t provide the same level of insulation, so I apply them extra thickly at 6 inches or more. The added soil nutrition and water retention are other big bonuses.

Quick Hoop Tunnels for Temporary Heat

When an unexpected cold snap threatens, I break out my emergency quick hoop tunnels as protection.

These very temporary covers involve flexible hoops made of thin PVC tubing or conduit wiring. I bend them over beds or rows and cover with clear plastic anchored by boards or rocks against wind.

Benefits include:

• Super fast and easy installation.
• Lightweight for storage and reuse.
• Allows ventilation when lifted temporarily.
• Prevents frost and freezing damage.
• Can self-regulate temperature on sunny days.

Quick hoops are my go-to emergency defense for short cold spells mid-season.

Extending the Growing Season for Specific Crops

Now that we’ve covered techniques, let’s look at which crops can actually survive and thrive with season extension into fall, winter and early spring:

Cool Weather Crops to Grow in Fall and Winter

Some vegetables actively grow in cool weather and can withstand a light frost. These include:

• Arugula
• Beets
• Broccoli
• Brussels sprouts
• Cabbage
• Carrots
• Cauliflower
• Collards
• Garlic
• Kale
• Leeks
• Lettuce
• Onions
• Peas
• Radishes
• Spinach
• Swiss chard

I succession sow these crops in late summer to keep the fall harvest going. With protection, some will overwinter for spring harvest.

Growing Salad Greens Year-Round

With protection from a greenhouse or coldframe, growing salad greens in winter is easy. I focus on:

Lettuces – Leaf, romaine and bibb lettuce handle low light.

Spinach – Slow bolting varieties overwinter well.

Arugula – The spicy greens grow quickly in cool conditions.

Mizuna – Mustardy Japanese greens stay mild in winter.

Kale – The superfood greens thrive year-round.

Swiss chard – Cold hardy with colorful stems.

I grow these densely in containers for frequent winter salads. Cold frames or hoop house covers prevent frost and keep them actively growing all season.

Overwintering Root Vegetables

Many root veggies can stay in the garden through winter for early spring harvest:

Carrots – Mulch heavily to prevent the ground from freezing.

Beets – Cover with straw or leaves over winter.

Radishes – Use low tunnels for protection.

Parsnips – Leave these sweet roots in the ground until spring.

Onions – Green onion bulbs will bounce back in spring.

I top the dormant roots with 2 feet of mulch in late fall. They sweeten with light frosts and offer early harvests come spring.

Fruiting Crops that Thrive in Cooler Weather

With protection, some fruiting veggies can grow outside of peak summer:

Strawberries – Use cloches to get early and late season berries.

Tomatoes – Start plants early and grow indeterminate varieties in a greenhouse.

Peppers – Lean towards shorter season early producers like Hungarian wax.

Eggplant – Look for fast maturing, cold tolerant varieties like Ichiban.

Peas – Sugar snap and snow peas flourish in cool conditions.

Bush beans – Use cloches or tunnels to extend spring and fall harvests.

Given the right covering, surprising amounts of tomatoes, peppers and eggplant can be coaxed from an unheated greenhouse through winter.

Herbs and Flowers for Year-Round Harvesting

A wide range of herbs and flowers relish prolonged cool temperatures:

Parsley – Extremely cold hardy and continues growing into winter.

Cilantro – Often survives winters if heavily mulched.

Dill – Best treated as an annual but may self sow.

Thyme – An evergreen perennial lasting for years.

Oregano – Also evergreen, oregano remains productive year-round.

Chives – Insulate the roots in very cold winters.

Calendula – This cheery flower blooms early and late season.

Snapdragons – Use tunnels or coldframes to protect these.

Violas – Mix with greens for spots of winter garden color.

Herbs and flowers lend variety to the winter harvest and provide pest control benefits when planted near vegetables.

Managing Pests When Growing Year-Round

Extending the garden season also extends potential interactions with pests. Here are tips to minimize problems:

• Use row covers to physically exclude pests like cabbage worms and flea beetles.
• Eliminate weed habitat around beds to deprive pests of shelter.
• Apply horticultural oils to suffocate overwintering eggs and larvae.
• Use autumn leaves as natural mulch rather than wood chips that harbor slugs.
• Remove and destroy seriously infested plants to prevent spread.
• Monitor for pests daily and hand-pick large bugs.
• Introduce beneficial insects like ladybugs and lacewings that control aphids.

With proactive prevention and early intervention, I’m able to keep pests from getting out of hand during the winter months.

Avoiding Disease Pressure in Enclosed Spaces

Growing in greenhouses and tunnels poses increased risk of fungal diseases and other plant pathogens. I reduce this by:

• Allowing maximum ventilation whenever possible.
• Not crowding plants to provide good airflow.
• Sanitizing any reused planting containers.
• Pruning diseased parts immediately and removing them from the garden.
• Allowing proper drying time between irrigating.
• Monitoring soil moisture levels – don’t overwater!
• Growing resistant or tolerant cultivars when available.
• Finish the season by removing and burning all plant debris.

With good cultural practices, crops thrive in protected environments while diseases are kept in check.

Tracking Sunlight and Optimizing Plant Spacing

When growing through shorter winter days, plants need every bit of sunlight available. I ensure they get enough by:

• Recording the sun’s shifting angles and adjusting covers or plant placement as needed.
• Removing any obstructions like tools or trellises that cast shadows.
• Spacing plants appropriately so they don’t shade each other.
• Pruning back taller crops to prevent shading of lower ones.
• Supplementing solar levels with full spectrum grow lights on overcast periods.

Maximizing every ray keeps my crops actively photosynthesizing and growing through the darker months.

Planning Crop Rotation for Continual Harvests

To keep the garden productive year-round, I group crops by planting time and nutritional needs to plan rotations that maximize space and yields.

Fall – Broccoli, lettuce, carrots, radishes, greens

Winter – Onions, garlic, spinach, chard, leeks, asian greens

Early spring – Peas, beets, cabbage, potatoes, cauliflower

Late spring – Beans, squash, cucumbers, tomatoes, eggplant

I schedule the rows so follow-on plantings replace each crop as it finishes. This avoids depleting soils and helps break disease cycles.

Conclusion: Get the Most out of Your Urban Garden

With the right combination of season extending techniques, warm weather crops in an unprotected outdoor garden can stretch into winter. For serious harvests year-round, enclosed structures like greenhouses allow maximum control and plant growth even in the coldest months.

The rewards of fresh produce and herbs in the heart of winter are well worth the extra effort. Urban gardeners can save money, increase self-sufficiency and enjoy garden-to-table goodness 12 months a year!

I hope these tips help you prolong the bounty beyond a single short season. Let me know which season extension methods work best for your unique climate and growing goals!