As a landscaping professional with over 20 years of experience, I‘ve helped countless clients successfully use galvanized metal tubs for raised garden beds and planters. While galvanized containers offer many benefits, I‘m often asked – are they truly safe for growing edible plants?
In my experience, galvanized tubs can be a smart, budget-friendly choice for gardening if used properly. The key is understanding how to minimize the potential downsides. In this comprehensive guide, I‘ll share everything I‘ve learned to help you make the right decision for your garden and use galvanized containers safely.
What Exactly is Galvanized Metal?
Let‘s start with a quick primer on what galvanized metal is and how it‘s made. Galvanized steel begins as regular carbon steel. It‘s then dipped into a bath of molten zinc. This coats the steel with a thin layer of zinc oxide.
The zinc bonds tightly to the steel to form a protective layer. It‘s this zinc outer coating that gives galvanized steel its silver-gray finish. The technical term for this process is "hot-dip galvanizing."
Galvanizing was first patented in 1837 by French chemist Stanislas Sorel. By the 1900s, galvanized metal became widely used for industrial and consumer products due to its weather resistance and durability.
Today, an estimated 50% of zinc production is used in galvanizing steel. Building and construction utilize around 50% of galvanized steel, while container manufacturing makes up around 4%.
Key Benefits of Galvanized Metal for Gardening
There are some clear advantages that make galvanized steel containers advantageous for many gardeners:
Extreme Durability
The zinc coating gives galvanized steel incredible durability and longevity. It adds a protective barrier that prevents rusting and corrosion far better than paint or other coatings.
In fact, studies show that galvanized steel retains its protective zinc barrier for an average of 40-70 years before corrosion begins to occur from environmental exposure.
The life span is even longer when galvanized containers are used for gardening since they aren‘t exposed to some harshest outdoor elements. Routine maintenance and upkeep can easily extend the life of a galvanized tub to 15-20 years or more.
This makes galvanized an extremely cost-effective investment over the long term. A quality galvanized tub can literally be the last one you ever need to buy.
Affordability
Speaking of cost, galvanized steel containers are very affordably priced, especially compared to other metal options.
For example, a new galvanized tub of similar dimensions can cost 60-75% less than stainless steel. This makes them accessible to most budgets.
You can also frequently find used galvanized containers at garage sales, thrift stores, or salvage yards for even lower prices. Repurposed galvanized tubs add vintage charm to gardens.
Strength & Stability
Another key advantage of galvanized steel is its unmatched strength. Galvanized tubs are practically indestructible when cared for properly.
They won‘t easily crack, shatter, or collapse like ceramic, wood, or plastic containers might. This makes them ideal for long-term, permanent placement.
I‘ve seen 30-year old galvanized containers still going strong! They can tolerate some rough treatment, occasional frost heaves, bumps from garden hoses, and more. Galvanized steel has a tensile strength of 50,000 psi.
The rigidity provides excellent stability and support for plants. Galvanized tubs are safe to move when filled with soil, unlike more flexible options.
Heat Absorption
Metals like galvanized steel absorb heat from the sun and reflect it back to plants and soil. This can warm root zones by as much as 10-15 ̊F compared to plastic or wood planters.
For crops that thrive in warm soil like tomatoes, peppers, and basil, galvanized containers have a clear advantage. The heat reflection can help extend the growing period in fall as well.
Non-Porosity
Galvanized steel has a perfectly smooth, non-porous surface unlike terra cotta or concrete. This prevents any moisture absorption issues.
As a result, galvanized containers have excellent drainage and are easier to keep an optimal moisture balance. The risk of overwatering is lower compared to porous containers.
Potential Downsides of Using Galvanized Containers
Of course, no material is perfect for every situation. There are some potential disadvantages to consider when deciding if galvanized is right for your garden:
Zinc Leaching
The primary concern with galvanized containers is the potential for zinc to leach into the soil as the protective coating deteriorates.
While zinc is an essential plant nutrient, excessive amounts can become toxic for sensitive crops. Root plants like carrots and potatoes are at highest risk.
However, modern galvanized coatings are engineered for maximal corrosion resistance. Studies show minimal zinc leaching within the expected 10-15 year lifespan if used for gardening.
One University of Kentucky study found lettuce grown in galvanized containers absorbed only 3x more zinc than lettuce in plastic containers – still well below toxicity levels.
Acidic Soil Reactions
The zinc in galvanized steel can react with acidic soils, causing the coating to erode faster. This accelerates the risk of excess zinc absorption.
For this reason, galvanized containers are best suited for soils with a neutral or slightly alkaline pH between 6.5-7.5. Acid-loving plants like blueberries are not ideal.
Test your garden soil‘s pH annually and amend with lime if needed to maintain a neutral level. Also, choose soilless potting mixes near 7.0 pH to fill containers.
Lead in Older Containers
Another concern is potential lead contamination in vintage galvanized containers made before the 1960s.
Up to 2% lead was added to the zinc coating in some early galvanized products. Always thoroughly inspect older galvanized containers for lead dust or paint, and test if unsure.
I strongly recommend only using newer galvanized containers made after 1970s regulations phased out lead use. Lead has no place around edible gardens.
Lack of Scientific Research
There is very little peer-reviewed research on growing food crops in galvanized containers. The long-term safety is not conclusively proven.
For example, only a handful of university studies have tested zinc absorption rates from galvanized steel into plants. Much more research is needed to quantify the risks.
As a result, many horticulture experts take a cautious approach in recommending galvanized for edible gardening.
Eventual Corrosion
While it takes decades, galvanized steel will start to exhibit corrosion as the zinc layer wears off from weather exposure.
This first appears as white zinc oxide spots, eventually turning to red iron oxide rust. Corrosion accelerates the potential for zinc leaching.
But again, with proper maintenance, a quality galvanized container should last at least 15 years even in harsh outdoor conditions before any noticeable corrosion if used for gardening.
Safety Recommendations for Galvanized Tubs
Based on my experience, galvanized containers can be utilized safely for gardens if the proper precautions are taken:
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Always inspect new tubs for defects and ensure they are lead-free. I recommend looking for tubs made after the year 2000 as they have better zinc coatings.
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Have your garden soil tested annually to maintain between pH 6.5-7.5. Adjust with lime if needed to prevent zinc reactions.
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Use a high-quality potting mix like FoxFarm Ocean Forest instead of garden soil. It‘s optimized for container gardening with proper pH and drainage.
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Drill extra drainage holes if they are not already provided. At minimum, add 4-6 holes spaced evenly across the bottom.
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Place 1-2 inches of pebbles or stones at the bottom of containers below soil to improve drainage and air flow.
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Water carefully to avoid any standing water or excessive moisture buildup.
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Fertilize regularly with organic liquid fertilizers to replenish nutrients that can leach out faster in containers.
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Test soil zinc levels once per year. Toxicity typically occurs above 70 ppm.
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Grow leafy greens and herbs that absorb less zinc than root veggies.
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Be diligent about inspecting containers annually for any corrosion or damage. Address any issues immediately.
Alternative Materials to Galvanized
If you prefer to avoid galvanized steel altogether, here are some other excellent container options:
Stainless Steel
Stainless steel is ideal if you want a durable metal – it resists corrosion better than galvanized and has a longer lifespan. Electropolished stainless steel is safest. Cost is quite a bit higher.
Food-Grade Plastic
Plastic pots like HDPE are lightweight and very affordable. However, they lose stability when soil is added and may need replacing every few years. Avoid BPA/PVC plastics.
Wood
Wood planter boxes allow you to customize sizes easily. Use naturally rot-resistant woods like cedar. Add waterproof liner to extend life. Avoid pressure treated.
Ceramic/Terra Cotta
Unglazed terra cotta has excellent natural drainage but is prone to cracking over time. Glazed ceramic is decorative but retains more moisture.
Concrete
Concrete planters are heavy but extremely durable. Use concrete rated safe for edible gardens. Can leach lime so test soil pH.
Fiberglass
Fiberglass boxes offer durability like metal with no leaching risk. However, material costs are higher. Avoid treated fiberglass.
Growing Successfully in Galvanized Tubs
Based on growing in galvanized containers for 20+ years, here are my top tips:
Location
Galvanized steel absorbs heat, so place containers where plants will get full sun for at least 6-8 hours per day. Avoid dense shade.
Southern or western exposure works best to take advantage of afternoon sun. You can also prop up one side with stones to angle toward sunlight.
Drainage
Ensuring adequate drainage is essential. Drill at least 4-6 holes evenly spaced across the entire bottom surface.
Make holes at least 1/4" diameter. You can always drill more holes later if needed. Use a sanding bit on a cordless drill for smooth edges.
Also add a 1-2 inch layer of gravel, pebbles, or wood chips at the bottom before filling with soil. This creates a drainage reservoir.
Soil Quality
Never use compacted garden soil, as it will constrain root growth. Always use a loose, lightweight potting mix formulated for containers.
I recommend FoxFarm Ocean Forest or Espoma Organic Potting Mix. They provide the ideal texture and pH around 6.5-7.0 for containers.
When filling pots, leave at least 2 inches of space below the rim for proper watering.
Fertilization
Container plants rely on frequent feeding since nutrients leach out readily. Use organic liquid fertilizers like fish emulsion or compost tea. Apply every 2-4 weeks from spring to fall.
Mix in a balanced granular organic fertilizer like Espoma Garden Tone when first potting plants. Top dress containers with compost or worm castings twice each season.
Water Needs
Check soil moisture daily, especially on hot, dry, or windy days. Galvanized steel dries out faster than other materials.
Water thoroughly when the top 1-2" becomes dry until excess drains from the bottom. Soil should remain evenly moist, not constantly soggy.
Use drip irrigation on a timer to ensure consistent moisture if you can‘t check manually. Mulch surface with wood chips to retain moisture.
Plant Selection
You can grow a huge variety of edible and ornamental plants in galvanized tubs! Focus on shallow rooted crops.
Best veggies are leafy greens, tomatoes, peppers, bush beans, broccoli, spinach, lettuce, beets, peas, and radishes.
Herbs thrive in galvanized like parsley, basil, oregano, sage, cilantro and thyme.
For flowers, choose small bulbs or annuals like marigolds, zinnias, cosmos, impatiens, begonias, pansies, and petunias. Make sure all sunlight and soil needs are met.
Maintaining Your Galvanized Containers
With the proper care, your galvanized tubs should last over 15 years. Here are my top maintenance tips:
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Inspect containers thoroughly each season for any cracks, perforations, deformities, or corrosion. Look along the inside seam and along bottom edges. Use 100 grit sandpaper to gently smooth any rough areas.
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Wash regularly by hand or with a low-pressure hose to remove dust, dirt, debris and salt deposits. Avoid harsh detergents. Make sure water can drain fully.
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Use a plastic scrub brush and mildly abrasive paste cleaner for tougher grime. Avoid scouring pads or steel brushes that can damage the zinc layer.
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Dry thoroughly after any rains or irrigation with a towel to avoid moisture buildup. Store containers flipped or covered during winter.
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For minor scratches or dents, use a zinc-based touch-up pen available at hardware stores.BUFFER
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To limit corrosion, brush edges annually with clear enamel sealant like Rust-Oleum Clear Coat. Avoid coated interior surfaces.
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Watch closely for any white spots indicating early zinc corrosion. Sand immediately and re-seal affected areas.
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If excessive corrosion occurs, replace galvanized containers within the season to prevent zinc leaching.
Troubleshooting Common Galvanized Tub Problems
Here are some common issues I‘ve seen come up with galvanized containers and how to address them:
Rust formation – First, determine if it is surface zinc oxide or actual iron oxide rust. Zinc corrosion can be scrubbed and sealed. Rust may require sanding to a smooth surface before sealing.
Water pooling – Improve drainage by drilling more holes and removing any packed soil or roots blocking holes. Ensure container is level.
Salt buildup – Salt deposits appear as white residue. Scrub with vinegar and water solution. Improve rinsing after fertilizing.
Discoloration – Sometimes galvanized can get "tannin stains" from acidic soils and fertilizers. Try scrubbing with baking soda paste.
Zinc nodules – Visible zinc clumps in soil indicate zinc leaching. Test soil and replace if excessive zinc. Avoid acidic soil and overwatering.
Pitting – Small holes from material deterioration or damage. Attempting sealing pits with zinc paint. May require replacing container.
Deformation – Can occur if galvanized steel freezes when holding water, creating bulges. Drain containers fully for winter.
Lead detection – Use lead-test kits to check vintage containers. Never use containers with lead or replace/encapsulate as needed.
When to Replace Your Galvanized Tubs
Here are some signs that it‘s time to retire those trusty old galvanized containers:
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Multiple large areas of corrosion and rust
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Holes larger than 1/4 inch
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Visible pitting, cracking, or deformation
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Soil zinc levels exceed 70 ppm
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Plants show toxicity symptoms like stunted growth
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Unsealable lead levels detected in vintage containers
For the longevity conscious, I recommend having a backup container ready to replace older galvanized tubs exhibiting wear after 15-20 years of use.
Conclusion: A Smart Choice With Precautions
In summary, galvanized steel containers offer numerous benefits that make them a versatile, durable, and budget-friendly choice. However, some potential risks like zinc leaching mean extra precautions should be taken.
Follow my safety and maintenance tips, test soil regularly, and properly retire aging containers. Used thoughtfully, galvanized tubs are a clever way to affordably and productively expand your gardening space for years to come.
I hope this comprehensive guide gives you confidence in determining if galvanized is the right material for your gardening needs and using galvanized containers successfully! Let me know if you have any other questions.
