Temperatures Are About to Spike – Take These 5 Crops Out of Your Greenhouse Now to Avoid Bolting
Get an even bigger harvest by relocating these homegrown goods
Adding a greenhouse to your garden is invaluable, offering a controlled environment to start seeds early. But knowing when to move plants outside is key to avoiding them bolting, i.e., going to flower prematurely. In May, when temperatures suddenly spike, many salad crops are at risk of bolting if left in the greenhouse.
There are so many vegetables to grow in your greenhouse in spring, which offers protection from lingering cold temps and allows you to get a head start. But, as warm spring temperatures settle in, the greenhouse can become overly hot – too hot for cool-season crops. In response to this heat and drought stress, arugula, spinach, and more put out flower spikes to set seed, no longer using their energy to produce the tasty leaves we desire. Once this happens, they often become less palatable.
1. Lettuce
If you started lettuce in your greenhouse, now is the time to relocate it before the hottest temperatures cause it to bolt.
'Lettuces are notorious for bolting once temperatures rise. Sometimes, you turn your back for five minutes, and a thick central stem appears,' says Drew Swainston, Content Editor at Homes & Gardens.
'Warm weather and dry conditions (like in a greenhouse) are the double threat that causes them to bolt.'
'If you started growing lettuces indoors earlier in the year to extend the season, move their containers to a shadier spot to prolong your harvests. And if you sowed lettuce seeds indoors but have yet to plant them outside, do it ASAP to give yourself any chance of a crop,' Drew continues.
It's best to plant them somewhere protected from the intense afternoon sun, and make sure to keep them well-watered.
It is also possible to purchase bolt-resistant lettuce varieties, including this ‘Heatwave Mix’ from Burpee.
Drew has worked as a writer since 2008 and was also a professional gardener for many years. As a trained horticulturist, he worked in prestigious historic gardens, including Hanbury Hall and the world-famous Hidcote Manor Garden. He also spent time as a specialist kitchen gardener at Soho Farmhouse and Netherby Hall, where he grew vegetables, fruit, herbs, and cut flowers for restaurants. Drew has written for numerous print and online publications and is an allotment holder and garden blogger. He was shortlisted for the Digital Gardening Writer of the Year at the 2025 Garden Media Guild Awards.
2. Arugula
Arugula is a cool-season crop you can start early in your greenhouse. But, leave it in there as temperatures warm up, and you'll soon see its foliage turn red (a stress response to excessive light and heat) and yellow flower spikes emerge.
This usually happens with consistent temperatures between 68-74°F and longer daylight hours. When it bolts, the flavor becomes very peppery, and the texture becomes tough. Although the blooms are also edible and can be added to salads for a delicate peppery flavor.
Move arugula containers outdoors now to somewhere with partial shade. Young plants started in the greenhouse can also be transplanted now, and you can start sowing seeds outdoors every couple of weeks until fall for a continuous supply through the growing season.
Regularly harvesting arugula from the outer leaves can also extend its season as a cut-and-come-again vegetable. Use these garden scissors from Amazon to make clean cuts when harvesting.
Find arugula seeds on Amazon.
3. Spinach
Growing spinach in a greenhouse in early spring allows for harvesting before the summer. But, this cool-season crop thrives in cooler temperatures (around 50-75°F) and will send up a flower spike very quickly when temperatures suddenly rise.
To stop your spinach bolting, move containers outdoors in May. You can then sow more spinach seeds directly outdoors before the summer heat arrives, and then again in the milder temperatures of late summer-fall.
Harvesting spinach leaves regularly also helps extend its season, as well as keeping it in a shaded spot and keeping it well-watered to reduce heat and drought stress.
Shop spinach seeds at Burpee.
4. Bok Choy
You can sow bok choy indoors in spring for a head start. But, keeping it in the greenhouse in May will only be counterproductive, as temperatures rising above 70°F will trigger flowering.
'Bok choy is a slug magnet, so I always sow mine into modules first and then plant out,' says gardening expert Stephanie Hafferty.
'Fill module trays (like these from Burpee) with peat-free multi-purpose compost and sow two seeds into each module. Thin out, leaving the strongest seedling in each module. Then plant out when the seedlings are about two inches tall. Keep bok choy well-watered in hot, dry weather,' she advises.
Shop bok choy seeds on Amazon.
Stephanie Hafferty is an award-winning garden writer, organic gardener, designer, food writer and educator, who has been growing her own food for more than 30 years.
5. Cilantro
Cilantro is another crop to move out of your greenhouse in May if you started seeds indoors in containers. It's prone to sending up a flower spike as soon as temperatures rise above 70-75°F and daylight hours are longer.
When it bolts, a white flower stalk appears, and the flavor of the leaves becomes less intense and more bitter.
Moving pots outdoors into a shady area is the best way to prolong its season and harvest, delaying the development of flowering until summer arrives.
Nevertheless, it can be good to leave some cilantro plants to flower, offering a valuable source of nectar for pollinators in your garden. Plus, you can collect seeds from the flowers for planting in the fall.
Shop cilantro seeds at Burpee.
You can get slow-bolt cilantro seeds on Amazon to delay flowering further.
What to Shop
This garden hod is great for harvesting homegrown goods. It has a mesh trug so you can rinse crops easily.
These bypass pruning shears are ideal for essential pruning tasks. They cut up to 1" in diameter.
These long-sleeved gardening gloves have a gorgeous floral pattern. They protect hands and arms when tending to the garden.
FAQs
Does Mulching Stop Bolting?
Mulching is key in delaying cool-season crops bolting because it keeps soil temperatures cool and retains moisture. Aim for a two-inch layer of organic mulch around the base of the plants, like this cedar mulch from Lowe's.
Early bolting is just one vegetable gardening mistake to avoid. You should also take care to space vegetables properly and choose the right crops for the growing space you have available.
If you are a novice GYO gardener, check out our guide to the easiest vegetables to grow to get started.
If you love inspiring garden ideas, outdoor advice, and the latest news, why not sign up for our newsletter and get the latest features delivered straight to your inbox?
