Are Old Tyres Safe to Use as Planters?

No study has tested vegetables grown in tyre planters, so nobody can say how much reaches a crop. Lab studies find that rubber leaches zinc and polycyclic aromatic hydrocarbons (PAHs), and a Washington State University specialist concludes rubber should not be used as a landscape amendment or mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition →. If you want to grow food, choose a different container.

What leaches

A 2022 review in the journal Polymers studied rubber crumb, the shredded rubber used in sports and playground surfaces, not whole tyres. Zinc was the metal released: dissolved zinc rose from 0.273 mg/L after 24 hours to 7.041 mg/L after 21 days, so leaching continues over time. The crumb contained PAHs totalling almost 58 mg/kg. Lead, copper and cadmium were not detected, which the authors link to EU restrictions on hazardous compounds in tyre production. The authors say the dissolution rate depends on the pH of the solution and on chloride ions.

In the review's toxicity tests, the earthworm Eisenia fetida showed 100% mortality after exposure to the crumb. Plants were less sensitive: root growth was inhibited by 31% in mustard (Sinapis alba) and 44% in lettuce (Lactuca sativa), with other plant tests between 27 and 48%. The authors raise the most concern about freshwater contamination, and say the impact on soil organisms has not been sufficiently studied. A solid tyre has far less exposed surface than crumb, so these figures do not transfer directly. Washington State University adds that rubber leachate contains a range of minerals including zinc, plus plasticisers and accelerators from manufacture, and that rubber exposed to road contaminants such as lead adsorbs them and releases them later. Illinois Extension notes that soil under rubber mulch has shown high zinc, which can be toxic to plants, with leaf yellowing and reduced tree growth.

What this means in Scotland

None of this has been tested on a tyre planter in a Scottish garden. It applies general findings to Scottish rainfall and soil, so treat it as a reason for caution, not a measured result.

If you still want to use tyres

Nothing here is tested on tyre planters, so this is precaution based on how contact works, not a measured result. Flowers and other ornamentals avoid the question, because nothing is eaten. For food, the root is the part in constant contact with the rubber, so potatoes, carrots and beetroot sit in the contact zone, leafy crops take up from the same soil and are eaten whole, and fruiting crops such as tomatoes develop furthest from the tyre wall. That ordering is our inference from how the plants grow, not a study. Lining the inside with plastic sheeting cuts soil contact with the rubber, and RHS advises the same lining for untreated timber frames, though plastic can itself release compounds. Painting is not shown to help (see the questions below).

Tyres do have a long-standing use for warming early potatoes in cold spring soil, because black rubber absorbs heat. It is long-standing allotment practice, and it means the crop most in contact with the rubber is the one the technique is used for.

The verdict

The leaching of zinc and PAHs from rubber is documented, and no study shows how much reaches a vegetable grown in a tyre. The sensible position is caution about eating anything grown in one. A commercial lab can test soil for heavy metals, but routine soil tests do not usually cover PAHs. A raised bed frame in another material removes the question: see the safer-alternatives answer below and raised beds against growing in the ground.

A cracked, weathered tyre can also expose its steel belting at the sidewall, a cut hazard worth checking whatever the chemistry.

Common questions

Sources