Growth plates in dogs close anywhere from 6 months to 24 months of age, depending primarily on breed size. Small breeds finish the fastest, while giant breeds like Mastiffs can take up to two years to reach full skeletal maturity. Understanding this timeline matters because open growth plates are vulnerable to injury, and the activities you allow your puppy to do should change once those plates have fully hardened into solid bone.
How Growth Plates Work
Growth plates are soft zones of cartilage near the ends of your puppy’s long bones. They’re the reason puppies grow taller. As your dog matures, bone-building cells gradually replace that cartilage with solid bone tissue. This process happens remarkably fast at each individual site: cells deposit new bone on a scaffold of mineralized cartilage, working simultaneously across multiple spots within the same plate so that a core sheet of bone forms quickly. Once that initial bone sheet is in place, it thickens and reshapes through a process where new bone is added on one side while old material is resorbed on the other.
The end result is dense, strong bone that replaces what was once a flexible, injury-prone cartilage zone. Until that replacement is complete, the growth plate remains the weakest structural point in your puppy’s skeleton, softer than the surrounding bone, ligaments, and tendons.
Timeline by Breed Size
The single biggest factor in when growth plates close is how large your dog will be as an adult. Smaller dogs reach skeletal maturity much earlier than larger ones.
- Small breeds (under 25 pounds): Growth plates typically close by 6 to 8 months of age. Breeds like Chihuahuas, Yorkies, and Toy Poodles fall into this category.
- Medium breeds (25 to 50 pounds): Most reach adult size around 12 months.
- Large breeds (50 to 70+ pounds): Growth plates close between 12 and 18 months. Labs, Golden Retrievers, and German Shepherds are common examples.
- Giant breeds (80+ pounds): Growth continues for 12 to 18 months, and some breeds like Mastiffs and Great Danes may not reach their fully grown size until 24 months.
These ranges cover overall growth. Individual bones and joints within the same dog close on their own schedule, which means your puppy’s skeleton doesn’t finish maturing all at once.
Which Bones Close First
Growth plates closer to the body’s center tend to close before those farther out. In the foreleg, for example, the upper (proximal) portion of the ulna fuses first, around 20 to 24 weeks of age. The upper radius follows at 24 to 28 weeks. The lower end of the ulna closes at 28 to 32 weeks, and the lower end of the radius is the last to finish, typically at 32 to 40 weeks.
This staggered closure is important. If one growth plate in a paired bone (like the radius and ulna, which run side by side in the foreleg) closes prematurely while the other stays open, the still-growing bone can push the limb out of alignment. That’s one reason growth plate injuries in puppies require prompt veterinary attention.
How Spaying and Neutering Affects Closure
Sex hormones play a direct role in signaling growth plates to close. When a dog is spayed or neutered before reaching skeletal maturity, the removal of those hormones can delay closure. The long bones may grow slightly longer than they otherwise would have, which can subtly alter joint alignment. Research published in Frontiers in Veterinary Science found that this change in bone length may be enough to increase the risk of joint disorders in some dogs.
This doesn’t mean early spaying or neutering is always harmful. The effect varies by breed and body size, and there are valid health reasons for early sterilization in many cases. But it’s one factor worth discussing with your vet, especially for large and giant breeds where growth plates stay open the longest and joint problems like hip dysplasia and cruciate ligament tears are already more common.
Exercise Limits While Plates Are Open
Because open growth plates are softer than mature bone, high-impact or repetitive stress can damage them. VCA Animal Hospitals recommends waiting until your puppy reaches skeletal maturity before introducing long jogs, hikes, jumping over hurdles, or competitive agility. For small dogs, that waiting period ends around 6 months. For giant breeds, you may need to hold off until about 18 months.
That doesn’t mean puppies should be sedentary. Short walks, free play in the yard, and gentle romping with other dogs are all appropriate and important for muscle development and socialization. The activities to limit are the ones that put heavy, repetitive force on joints: sustained running on hard surfaces, leaping from heights, and forced exercise where the puppy can’t stop when tired. A good rule of thumb is to let your puppy set the pace and take frequent breaks.
Nutrition and Growth Plate Health
What your puppy eats directly affects how growth plates develop. The two minerals that matter most are calcium and phosphorus, and the ratio between them is more important than the absolute amount. The recommended ratio for growing puppies is about 1.2 to 1.4 parts calcium for every 1 part phosphorus. Both too little and too much calcium can cause problems. Excess calcium, particularly in large breed puppies, has been linked to skeletal abnormalities.
Caloric intake matters too. Even mild overfeeding can accelerate growth rate without giving the skeleton time to develop properly. Research has shown that even puppies raised on restricted energy supply can develop premature growth plate closure, suggesting the system is sensitive in both directions. Feeding a puppy food formulated specifically for your dog’s expected adult size is the simplest way to get the balance right. Large-breed puppy formulas, in particular, are designed with controlled calcium and calorie levels.
Signs of a Growth Plate Injury
Growth plate fractures are more common in puppies than in adult dogs, simply because the growth plate is the structural weak link. The most common signs are sudden lameness, swelling near a joint, pain when the limb is touched or moved, and reluctance to bear weight on the affected leg. A puppy may yelp during play and then hold a leg up afterward.
These injuries are diagnosed through X-rays. A veterinarian looks for shifts in the position of the bone end relative to the shaft, changes in the density of the growth plate zone, or a characteristic triangular bone fragment near the plate. Because the cartilage itself doesn’t show up well on X-rays, the diagnosis sometimes depends on subtle alignment changes rather than an obvious break.
Growth plate fractures need treatment quickly. If the damaged plate heals unevenly or closes prematurely on one side, the bone can grow crooked or stop growing altogether, leading to a permanent limb-length difference or angular deformity.
How Vets Confirm Growth Plates Are Closed
If you’re wondering whether your dog’s growth plates have fully closed, the only definitive answer comes from X-rays. On a radiograph, an open growth plate appears as a dark line or gap near the end of the bone because cartilage doesn’t absorb X-rays the way bone does. As the plate closes, that dark line gradually narrows and eventually disappears, replaced by solid bone that blends into the surrounding structure. Once no visible gap remains, the plate is considered fully fused.
This is particularly useful before starting a dog in high-impact sports or intensive training. If your large-breed dog is 12 months old and you’re eager to start agility or running, a quick set of X-rays can tell you whether the key joints are ready.