Planting spring bulbs in pots and planters

Everything I've learned about growing bulbs in pots but keeping it all very simple.

Spring bulbs truly lift our spirits after a long winter. Their oh so cheery dispositions break the monotony of greens and browns and suddenly, we have colour! A wondrous transition from monochrome to technicolour. I’m a huge fan! I plant up over 30 pots and planters for the Terrace Garden and Courtyard Garden.

Container displays have one huge advantage over planting in the ground… They can be moved. I know that’s obvious, duh! But, by planting for the whole spring season and shuffling those pots around, you can keep the display looking its absolute best for months. When those early flowering crocus, iris, and early daffs are fading you don’t need to keep looking at withering flowerheads… Swap them out for the musacri, richly coloured tulips, majestic crown imperials, and pom-pom alliums. With a little forethought in planning and planting, you can have a display in bloom from February right through to June. When the bulbs have faded, turf them out (hot-swapping) and use the same pots for your summer displays. What’s not to love?!

What you’ll find in this post…

Below, you'll find my best tips and advice for potting spring flowering bulbs in containers: Planting depths, potting mixes, crocks, winter protection, plus watering and feeding. There is a wealth of information out there about how to plant bulbs in pots, planters, or any container you wish. Sadly, most of it is over complicated and unnecessarily convoluted and it needn't be. Potting up bulbs is very simple. It is easy gardening! Essentially, you need a container that drains well, gritty potting compost that also drains well, and one general rule for planting depths. That's it.

What do I mean by drains well? Answer: Water needs to be able to pass freely through the compost and out the base of the container. Waterlogged compost is an absolute No!No! Waterlogging leads to rotting and flowering will be poor if the bulbs survive at all. Unfortunately, in waterlogged soils, almost all bulbs will rot and perish before ever seeing the light of day and that's the single most important tip you need to know.

Bulb compost (potting) mix

My bulb potting compost is a blend of SylvaGrow peat-free multi-purpose compost, sieved garden compost, and sieved leaf mould. The final ingredient is horticultural grit for increasing drainage. I make this up in batches, in a wheelbarrow. It’s not scientific and probably overly alkaline (shock, horror!) But it seems to work very well! The rough measurements are 50-30-10-10 which translates to:

1. Half bag of SylvaGrow peat-free multipurpose compost

2. One bucket of garden compost

3. Half bucket of leafmould

4. Half bucket of horticultural grit

5. Three standard scoops (70g) of Fish-Blood-Bonemeal (NPK 3-9-3)

I mostly eyeball my compost mix as garden compost and leafmould varies in consistency from year to year, season to season. Rubbing it through my fingers, I know when the overall blend is right. It shouldn’t stick together when you squeeze it in your hand. It should be very open and crumbly. You do get a feel for these things.

Having your own garden compost and leafmould is wonderful. But a high quality, shop-bought bark-based peat-free compost mixed with plenty of grit will produce great results! Remember, you need water to moisten the compost but pass through without waterlogging. Roots source the water and nutrition to fuel the leaves, leaves power the growth and development of the bulb.

If you want to grow the bulb and pack it full of nutrients for the following season, its best to give your bulbs that quality compost with a little extra fertility - either Fish-Blood-Bonemeal or liquid seaweed feed. If you’re growing bulbs as annuals, you don’t really need to be overly concerned with nutrition.

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To crock or not to crock, that's the question!

Previously, I’ve always advised using crocks - broken bits of pots or tile placed in the base of the pot to aid drainage and to stop the compost washing out the bottom. However, recent research by Which? Magazine has debunked this practice as pure dogma passed down by generations of gardeners. A layer of grit or sand evidently works much better and thankfully I have often recommend that too because how many of us actually have broken pots lying around?

Gravel and shingle are never far away in my garden and I simply grab a couple of handfuls to line the base of the pot. In theory, if your compost is gritty and free draining and your pot has plenty of drainage holes, waterlogging should not be an issue anyway.

Read the full Which? article here.

Bulb planting depths

A quick online search will show you a range of very elaborate depth gauges and graphics showing the precise depth of each individual species of autumn planted bulbs. But here's the thing... they make it so incredibly complicated! And, again, it shouldn't be! I follow a general guiding rule that bulbs are planted at a depth roughly 3x their own height as a minimum. So, if a bulb is 4cm (roughly 1½in), you plant it 12cm deep (about 5 inches). Don't make it complicated. For temporary planting in pots, the planting depth is arbitrary, so put the ruler away! You need a decent amount of compost below for the roots (say 15cm or 6in), and half as much above.

Interestingly, if bulbs weren't already remarkable enough, many bulbs actually have 'smart' roots. Yes! You did read that right. Many flowering bulbs have 'contractile' roots and are able to adjust their depth, if they find themselves planted too shallow... The wonder of bulbs continues!

"A recent study published in the Journal of the American Society for Horticultural Science proved that bulbs can adjust their planting position by moving deeper into the ground, apparently in search of moister, more conducive growing conditions." American Society for Horticultural Science. "'Smart' Flower Bulbs Pull Themselves To Deeper Ground." ScienceDaily, 10 December 2007 - Read in full here

If you really do prefer to geek out and like nothing better than measuring bulb depths to the exact millimetre, here's a list of common bulbs and depths. Notice how many actually say 'three times their depth'.

• Snowdrops: 10cm or three times their depth

• Crocus: 10cm or three times their depth

• Daffodils: 15cm or three times their depth

• Hyacinths: 10cm or twice their depth

• Tulips: 20cm or three times their depth

• Fritillaria imperialis, crown imperials: 30cm or three times their depth

• Globe alliums: 30cm or three times their depth

• Allium sphaerocephalon: 15cm or three times their depth

• Camassia: 10-15cm deep, or around twice the height of the bulb

The bulb planting method

Add a crock or layer of grit/gravel into the base as a drainage layer. Part fill your pot with roughly 15cm (6 inches) of compost, giving plenty of room for roots and lots of nourishment for the bulbs. Place a layer of bulbs on the compost, pointy end upwards, the hairy tuft of roots facing down. Top up with compost leaving a 2-3cm gap below the rim. Label. Done. Easy. But read on for the finer methods and a little more knowledge...

Bulbs planted in the ground have access to the threadlike 'wood wide web' of mycorrhizal fungi and the soil biome. Bulbs in pots however are usually planted in sterile compost from a bag, which is one reason I add garden compost and leaf mould. Many home gardeners won’t have access to homemade compost but you can add mycorrhizal fungi to pots using Empathy’s Bulb Starter or mycorrizal RootGrow. Just sprinkle a handful on the compost before placing your bulbs.

Now for your bulbs…

Once, you've added your drainage layer, pour in the compost and firm down. Sprinkle over the Bulb Starter or mycorrhizal fungi granules. Place the bulbs in the pot, pushing them firmly into the compost. I find concentric circles work nicely and they’re easy. No need for advanced mathematics here. I space-out bulbs just one finger-width apart. It may look too close, but trust me, it is very effective and with so many bulbs in one pot you get a really punchy show of blooms. For example, with a standard bucket-sized pot, I find 12-15 tulip bulbs will produce a great show. Just make sure the bulbs are not actually touching.

Now cover the bulbs with more compost, pushing the compost gently in between the bulbs, so there are no voids or gaps that might fill with water and rot the bulbs. Be gentle so you don’t overturn the bulbs, although their shoots are quite adept at finding their way to the surface. Make sure there is a gap of roughly 2-3cm (1 in) at the top, below the pot rim, so you can water easily without spilling over the top.

Finishing touches

Although not essential, a topping of grit is very useful. In the spring, weed seeds find it difficult to germinate and any that do are very easy to pluck out. When watering, the grit stops the compost sloshing about and making a mess. Just aesthetically, the fresh green stems and spring blooms look great set off against the grit. We’re gardeners. It’s ok to be fussy and make things pretty!

Finally, label the pot! This is important! You'll know what to expect in the new year and, crucially, keep a note on which cultivars worked for you, so you can order more next season. With so many colours and forms available (especially for narcissus and tulips) it great to have a mainstay or regulars while leaving room to try a few new cultivars each season.

If the pots are to go under cover for winter, water them first. Give those bulbs and compost a good soaking and let them drain. They probably won't be watered again until the spring. Make sure you can see water flowing out of the base so you know the drainage is working. If the pots are to stand outside in the elements, you can still water the pots, but just enough to settle the compost. The winter rains will do the rest.

How to protect your bulbs over winter?

If your winters are mild, like here in the South of England where we rarely see temperatures fall below -5℃ (23℉), you can leave your potted bulbs outside all winter. However, you might want to consider tucking them against your house because of the incessant rain! In the UK, the vast majority of winter rains blow in from the south west, so tucking those pots up against the north or east side of your house, keeps them out of the worst of any winter deluge.

Bulbs are very hardy, so no need to worry about frost, snow, or ice. They absolutely detest waterlogged soils/compost. Even when the Beast from the East locked the UK in ice for weeks on end (temperatures down to -18℃ (0℉), potted bulbs in my garden were fine, because the free draining compost wasn’t waterlogged. It's the combination of waterlogged soils that freezes solid that kills your bulbs.

During the winter, my pots all sit on gravel so they can drain freely. If your pots sit on flat patio paving, invest in pot feet, or make small chocks from wood, to lift the pots off the ground. Those pots must be able to drain. I cannot emphasise this enough!

Pests?

If you have problems with rodents eating your prized bulbs, lay sections of chicken wire or metal wire fencing over the pots and weigh these down with bricks. Do not be tempted to cover the pots with anything solid like planks of wood as this will reduce airflow and promote mould and fungal growth. You'll also be creating a cosy environment for slugs that will happily feast on any shallow-planted bulbs. When you see fresh shoots nosing through, you can remove the metal guards. Here, in my garden, the rodents seem far less interested when the bulbs are actively growing.

Signs of life

Once you see those green shoots pushing through, move your pots out to their final position as those bulbs will now need sunlight. As you're moving them, you can also check moisture levels. If the pots are really light, the compost is dry, so give them a thorough soaking. If they feel heavy, leave them and just double check they can drain!

Watering & Feeding

It’s difficult to recommend a general watering rule, now that Spring is so changeable. One year we have a borderline drought, the next it floods. Once bulbs are in growth, keep checking the compost by lifting the pot or pushing a finger in as deep as you can and seeing if there is still moisture there. Remember, most peat-free looks dry on top but retains moisture below.

I water our containers weekly. Giving them a through drenching so water begins to drain out the bottom. This seems sufficient, unless we get one of those baking springs, then I increase watering to twice weekly. Most bulbs will tell you they're thirsty by drooping. The flower stems will droop and the leaves buckle and crinkle.

Many gardeners swear by feeding their bulbs liquid seaweed feed all season, as soon as they’re in leaf. I’m not against this, but I’m not entirely convinced either. Where I can see feeding would be of benefit is just as the flowers fade and the bulb is still in full leaf, i.e. the Recharge Cycle aka Regeneration. Rather than feeding all season long, because seaweed feed is not exactly cheap, let’s use it where it’s most effective. During the bulb’s recharge cycle, it’s packing in more nutrients and growing the bulb, forming a new flower, before dormancy and life again next season.

More tips...

1. Decide on a planting scheme before you buy your bulbs. Are you going to have pots of mixed colours and mixed bulbs? Or one colour/type per pot? Complex vs Simple planting styles. My approach is very much on the simple side. One container, packed with one variety of bulb. No lasagne. No multi-bulb sandwiches. No faffing. Just one layer of bulbs. Colour schemes change from year to year, whether it's cool and elegant with whites, pastel pinks, and purples, or spicier with oranges, burgundies and blacks, hot pinks, and bold yellows. The White Garden style is always timelessly elegant!

2. Consider the eventual plant size versus pot size. It would look rather odd having a huge imperial crown fritillary in a tidily pot! As well as the overall size of the plant, bulbs need space below to send down roots and fuel their development. In small pots (half a bucket or 4-5ltr) I plant, crocus, reticulate iris, muscari, and dwarf narcissus. The larger narcissus, tulips, alliums, and imperial fritillary all go in bucket-sized pots and planters as a minimum.

3. Drainage is absolutely key.  The fear is always that bulbs rot away in waterlogged compost, without ever seeing the light of day. I really like vintage galvanised or copper planters as I can drill extra drainage holes. The buckets are especially useful as they're so easy to move about thanks to the bail (handle). Ceramics and most terracotta pots are limited to just one hole and its difficult to drill new holes through either material without breakages. Besides, I find ceramics particularly heavy and awkward to move. If you have mobility issues, consider plastic pots or metal buckets with handles.

Coming up…

In the next instalment of my Spring Bulb series, I’ll be looking at planting methods for lawns and borders, tools for the job, plus perennial planting companions for your bulbs and problems such as Tulip Fire and pests.